Radio terminal, radio communication system, and radio base station

ABSTRACT

A radio terminal comprises a communication unit, a measurement unit, a measurement unit, and a control unit. The communication unit selects the CDMA or OFDM scheme as a communication scheme to be used in a cell formed by a radio base station and performs radio communication. The measurement unit measures the receiving quality relating to the time difference between the preceding and delayed wave of a received signal in the OFDM scheme and measures the receiving quality of a received signal in the CDMA scheme. When the OFDM scheme is used in an idle state, when the receiving quality relating to the time difference which is obtained by the measurement unit is deteriorated below a threshold value corresponding to a guard interval used in the OFDM scheme, the control unit causes the measurement unit to measure the receiving quality of the received signal in the CDMA scheme.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. application Ser. No.13/260,125 which is the U.S. National Stage application of PCTapplication PCT/JP2010/055430 filed on Mar. 26, 2010, which claimspriority under 35 U.S.C. §119 to Japanese Patent Application No.2009-077744 filed on Mar. 26, 2009, Japanese Patent Application No.2009-127064 filed on May 26, 2009, Japanese Patent Application No.2009-127068 filed on May 26, 2009, and Japanese Patent Application No.2009-269471 filed on Nov. 27, 2009. The contents of each of theseapplications are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a radio terminal, a radio communicationsystem, and a radio base station which employ a CDMA scheme or an OFDMscheme.

BACKGROUND ART

Currently, 3G (or 3.5G) cellular phone systems employing a CDMA (CodeDivision Multiple Access) scheme are widely used.

In recent years, an OFDM (Orthogonal Frequency Division Multiplexing)scheme to transmit data in parallel by using multiple subcarriersorthogonal to each other has drawn attention as a new communicationscheme. The OFDM scheme is able to exert higher communicationperformances than the CDMA scheme, and is therefore used for 4G (or3.9G) cellular phone systems and the like.

Since installation of radio base stations supporting a new communicationscheme in a radio communication system such as a cellular phone systemgradually advances, it is expected that radio terminals supporting bothof the CDMA scheme and the OFDM scheme will be widely used in atransitional period from the third generation to the fourth generation.

Conventionally, a radio terminal supporting two communication schemes(so-called a dual terminal) switches a communication scheme used forradio communication with a radio base station (hereinafter referred toas a used communication scheme) by using the following method. Forexample, the dual terminal compares reception powers measured for therespective communication schemes and switches the used communicationscheme from the communication scheme with lower reception power to thecommunication scheme with higher reception power (see Patent Document1).

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: Japanese Patent Application Publication No.    2009-500956

SUMMARY OF INVENTION

A receiver side in the radio communication system receives a compositewave including multiple radio waves (multipath waves) taking differentpaths from a sender side. For this reason, in the OFDM scheme, thesender side adds a guard interval for absorbing a time differencebetween a preceding wave and a delayed wave to each OFDM symbol.

However, if a delayed wave delays for longer than a time length of theguard interval, OFDM symbols successive in time sequence in a receptionsignal of a receiver side causes an interference (so-called anintersymbol interference) and a communication performance is therebydegraded.

Here, the conventional dual terminal simply selects the communicationscheme with higher reception power. Accordingly, if the reception powerin the OFDM scheme is higher than that in the CDMA scheme, the OFDMscheme is used. However, there is a problem that if the intersymbolinterference occurs in the OFDM scheme, the OFDM scheme fails to exertthe intrinsic communication performance and only exerts a lowercommunication performance than the CDMA scheme. In addition, in light ofthe intersymbol interference, there is a problem that even though thedual terminal establishes connection by switching to a CDMA radio basestation due to the intersymbol interference, the dual terminal isaffected by the intersymbol interference again if switching to the OFDMscheme in the same location as the switching location.

Accordingly, it is an objective of the present invention to provide aradio terminal, a radio communication system, and a radio base station,which are capable of preventing deterioration in a communicationperformance due to an intersymbol interference.

The feature of a radio terminal according to the present invention issummarized as a radio terminal comprising: a communication unit(communication unit 1120) configured to perform radio communication byselecting any of a CDMA scheme and an OFDM scheme as a usedcommunication scheme to be used in a cell formed by a radio basestation; a first measurement unit (measurement unit 1140A) configured tomeasure reception quality related to a time difference (Tdmax) between apreceding wave and a delayed wave of a reception signal of the OFDMscheme; a second measurement unit (measurement unit 1140B) configured tomeasure reception quality of a reception signal of the CDMA scheme; anda control unit (control unit 1160A) configured to cause the secondmeasurement unit to measure the reception quality of the receptionsignal of the CDMA scheme when the OFDM scheme is selected as the usedcommunication scheme in an idle state and when the reception qualityrelated to the time difference obtained by the first measurement unit islowered to less than a threshold corresponding to a guard interval (Tg)used in the OFDM scheme.

According to the above-described feature, the control unit causes thesecond measurement unit to measure the reception quality of thereception signal of the CDMA scheme when the reception quality relatedto the time difference between the preceding wave and the delayed wavedeteriorates below the threshold. Therefore, it is possible to preparefor switching to the CDMA scheme in a situation where an intersymbolinterference is deemed to occur. In the meantime, since the OFDM schemeis used until the reception quality related to the time differencebetween the preceding wave and the delayed wave deteriorates below thethreshold, it is possible to establish a state where the communicationperformance of the OFDM scheme can be exerted. Accordingly, there isprovided the radio terminal which can prevent deterioration in thecommunication performance due to an intersymbol interference whileutilizing the OFDM scheme when the radio terminal supports both of theCDMA scheme and the OFDM scheme.

The feature of a radio communication system according to the presentinvention is summarized as a radio communication system (radiocommunication system 2010) comprising: a first radio base station (radiobase station 2200) supporting an OFDM scheme; a second radio basestation (radio base station 2300) supporting a CDMA scheme; and a radioterminal (radio terminal 2100) supporting both of the OFDM scheme andthe CDMA scheme and connected to the first radio base station, whereinthe radio terminal comprises: a terminal reception unit (reception unit2124) configured to receive signals of the CDMA scheme and the OFDMscheme; a first measurement unit (OFDM measurement unit 2141) configuredto measure a reception parameter indicative of a time difference betweena preceding wave and a delayed wave of a reception signal of the OFDMscheme; a second measurement unit (measurement unit 2142) configured tomeasure CDMA reception quality being reception quality of a receptionsignal of the CDMA scheme; and a terminal transmission unit(transmission unit 2122) configured to transmit, to the first radio basestation, the reception parameter measured by the first measurement unitand the CDMA reception quality measured by the second measurement unit,the first radio base station comprises: a base-station reception unit(reception unit 2224) configured to receive the reception parameter andthe CDMA reception quality from the radio terminal; and a base-stationtransmission unit (transmission unit 2222) configured to transmit, tothe radio terminal, an instruction for a handover to the second radiobase station when a value corresponding to the reception parameterreceived by the base-station reception unit exceeds a thresholddetermined based on the guard interval used in the OFDM scheme and whenthe CDMA reception quality received by the base-station reception unitis favorable.

According to the above-described feature, it is possible to preventdeterioration in the communication performance due to an intersymbolinterference while utilizing the OFDM scheme when using the radioterminal supporting both of the CDMA scheme and the OFDM scheme.

The feature of a radio base station according to the present inventionis summarized as a radio base station (radio base station 2200) to whicha radio terminal (radio terminal 2100) supporting both of a CDMA schemeand an OFDM scheme is connected, the radio base station supporting theOFDM scheme, comprising: a base-station reception unit (reception unit2224) configured to receive, from the radio terminal, a receptionparameter indicative of a time difference between a preceding wave and adelayed wave of a reception signal of the OFDM scheme and CDMA receptionquality being reception quality of a reception signal of the CDMAscheme; and a base-station transmission unit (transmission unit 2222)configured to transmit, to the radio terminal, an instruction for ahandover to a radio base station supporting the CDMA scheme when a valuecorresponding to the reception parameter received by the base-stationreception unit exceeds a threshold determined based on a guard intervalused in the OFDM scheme and when the CDMA reception quality received bythe base-station reception unit is favorable.

The feature of a radio terminal according to the present invention issummarized as a radio terminal (radio terminal 2100) supporting both ofa CDMA scheme and an OFDM scheme and connected to a radio base station(radio base station 2200) supporting the OFDM scheme, comprising: aterminal reception unit (reception unit 2124) configured to receivesignals of the CDMA scheme and the OFDM scheme; a first measurement unit(OFDM measurement unit 2141) configured to measure a reception parameterindicative of a time difference between a preceding wave and a delayedwave of a reception signal of the OFDM scheme; a second measurement unit(CDMA measurement unit 2142) configured to measure CDMA receptionquality being reception quality of a reception signal of the CDMAscheme; and a terminal transmission unit (transmission unit 2122)configured to transmit, to the radio base station, the receptionparameter measured by the first measurement unit and the CDMA receptionquality measured by the second measurement unit.

The feature of a radio base station according to the present inventionis summarized as a radio base station (radio base station 3200) to whicha radio terminal (radio terminal 3100) is connected, the radio terminalsupporting a predetermined communication scheme (next-generationcommunication scheme) that is any of an OFDM scheme and a SC-FDMA schemeand also supporting a CDMA scheme, the radio base station supporting thepredetermined communication scheme, comprising: a reception unit(reception unit 3224) configured to receive a signal of thepredetermined communication scheme from the radio terminal; ameasurement unit (measurement unit 3250) configured to measure areception parameter indicative of a time difference between a precedingwave and a delayed wave of the signal received by the reception unit,and a transmission unit (transmission unit 3222) configured to transmit,to the radio terminal, an instruction for a handover to a radio basestation supporting the CDMA scheme when the reception parameter measuredby the measurement unit exceeds a threshold determined based on a guardinterval used in the predetermined communication scheme.

According to the above-described feature, it is possible to preventdeterioration in the communication performance due to an intersymbolinterference, in the case of using the radio terminal supporting theCDMA scheme and the given communication scheme (the next-generationcommunication scheme) serving as either of the OFDM scheme and theSC-FDMA scheme.

A radio terminal according to the present invention is a radio terminalcapable of communicating with an OFDM radio base station supporting anOFDM scheme and communicating with a CDMA radio base station supportinga CDMA scheme, comprising: a control unit configured to control theradio terminal in a way that, when the control unit determines that theradio terminal is moved after connection to the OFDM radio base stationhaving an intersymbol interference is switched to connection to the CDMAradio base station, the radio terminal switches to connection to theOFDM radio base station.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an overall schematic diagram showing a radio communicationsystem according to a first embodiment.

FIG. 2 is a view for explaining a state of a composite wave to bereceived by a radio terminal according to the first embodiment.

FIG. 3 is a view for explaining a guard interval in the OFDM scheme.

FIG. 4 is a block diagram showing a configuration of the radio terminalaccording to the first embodiment.

FIG. 5 is a flowchart showing operations of the radio terminal accordingto the first embodiment.

FIG. 6 is a view for explaining waveform measurement processing by ameasurement unit according to the first embodiment.

FIG. 7 is a view for explaining EVM measurement processing by themeasurement unit according to the first embodiment.

FIG. 8 is a block diagram showing a configuration of a radio terminalaccording to a second embodiment.

FIG. 9 is a block diagram showing a configuration of a radio basestation according to the second embodiment.

FIG. 10 is a sequence chart showing operations of the radio terminal andthe radio base station according to the second embodiment.

FIG. 11 is an overall schematic diagram of a radio communication systemaccording to a third embodiment.

FIG. 12 is a view for explaining an OFDM signal to be received by aradio terminal according to the third embodiment.

FIG. 13 is a view for explaining a guard interval in the OFDM scheme.

FIG. 14 is a block diagram showing a configuration of the radio terminalaccording to the third embodiment.

FIG. 15 is a block diagram showing a configuration of a radio basestation according to the third embodiment.

FIG. 16 is a view for explaining EVM and an EVM threshold according tothe third embodiment.

FIG. 17 is a sequence chart showing an operation pattern 1 of the radiocommunication system according to the third embodiment.

FIG. 18 is a sequence chart showing an operation pattern 2 of the radiocommunication system according to the third embodiment.

FIG. 19 is a view for explaining waveform measurement processingaccording to a modified example of the third embodiment.

FIG. 20 is an overall schematic diagram showing a radio communicationsystem according to a fourth embodiment.

FIG. 21 is a view for explaining a SC-FDMA signal to be received by aradio base station according to the fourth embodiment.

FIG. 22 is a view for explaining a guard interval.

FIG. 23 is a block diagram showing a configuration of the radio terminalaccording to the fourth embodiment.

FIG. 24 is a block diagram showing a configuration of a radio basestation according to the fourth embodiment.

FIG. 25 is a view for explaining EVM and an EVM threshold according tothe fourth embodiment.

FIG. 26 is a sequence chart showing an operation pattern 1 of the radiocommunication system according to the fourth embodiment.

FIG. 27 is a sequence chart showing an operation pattern 2 of the radiocommunication system according to the fourth embodiment.

FIG. 28 is a view for explaining waveform measurement processingaccording to a modified example of the fourth embodiment.

FIG. 29 is an overall schematic diagram showing a radio communicationsystem according to a fifth embodiment.

FIG. 30 is a view for explaining an OFDM signal to be received by aradio terminal according to the fifth embodiment.

FIG. 31 is a view for explaining a guard interval in the OFDM scheme.

FIG. 32 is a block diagram showing a configuration of the radio terminalaccording to the fifth embodiment.

FIG. 33 is a block diagram showing a configuration of a radio basestation according to the fifth embodiment.

FIG. 34 is a view for explaining EVM and an EVM threshold according tothe fifth embodiment.

FIG. 35 is a sequence chart showing an operation pattern 1 of the radiocommunication system according to the fifth embodiment.

FIG. 36 is a sequence chart showing an operation pattern 2 of the radiocommunication system according to the fifth embodiment.

FIG. 37 is a view for explaining waveform measurement processingaccording to a modified example of the fifth embodiment.

FIG. 38 is a sequence chart showing an operation pattern 3 of the radiocommunication system according to the fifth embodiment.

DESCRIPTION OF EMBODIMENTS

First to fifth embodiments and other embodiments of the presentinvention will be described with reference to the accompanying drawings.In the following description of the drawings, same or similar referencesigns denote same or similar elements and portions.

First Embodiment

In a first embodiment, (1) Outline of Radio Communication System, (2)Configuration of Radio Terminal, (3) Detailed Operations of RadioTerminal, and (4) Operation and Effect will be described.

(1) Outline of Radio Communication System

FIG. 1 is an overall schematic diagram showing a radio communicationsystem 1010 according to a first embodiment. As shown in FIG. 1, theradio communication system 1010 includes a radio terminal 1100A, a radiobase station 1200A, and a radio base station 1300.

The radio terminal 1100A supports both of a CDMA scheme and an OFDMscheme. In FIG. 1, a cellular phone terminal is indicated as an exampleof the radio terminal 1100A. However, the radio terminal 1100A is notlimited only to the cellular phone terminal but may be a terminalequipped with communication devices according to the CDMA scheme and theOFDM scheme, for example. The radio base station 1200A supports the OFDMscheme. Note that an OFDMA (Orthogonal Frequency Division MultiplexingAccess) scheme is assumed to be included in the OFDM scheme in thisembodiment. Meanwhile, the radio base station 1300 supports the CDMAscheme such as a cdma 2000 scheme or a W-CDMA scheme. Each of the radiobase stations 1200A and 1300 forms a cell which is a communication area.

The radio communication system 1010 has a structure which is based onthe LTE (Long Term Evolution) standardized by 3GPP (3rd GenerationPartnership Project), for example. In the LTE, the OFDM scheme is usedfor downlink communication while the SC-FDMA (Single-CarrierFrequency-Division Multiple Access) is used for uplink communication. Inthe following, the downlink communication will be mainly described.

The OFDM scheme is the scheme configured to distribute data to multiplesubcarriers that are orthogonal to one another and then to modulate thesubcarriers. A sender side generates an OFDM signal by subjecting thesubcarriers to multi-phase PSK modulation or multi-value QAM modulationand then subjecting the subcarriers to inverse fast Fourier transform(IFFT). A receiver side performs demodulation by subjecting the OFDMsignal to fast Fourier transform (FFT).

When the radio terminal 1100A and the radio base station 1200A performradio communication in an environment unable to see each other directly,for example, an antenna of the radio terminal 1100A receives multipleradio waves (multipath waves) taking different paths as shown in FIG.2(a). In the example of FIG. 2(a), a path P1 directly reaching theantenna of the radio terminal 1100A and paths P2 and P3 reaching theantenna of the radio terminal 1100A after reflection by a building orthe ground are formed between an antenna of the radio base station 1200Aand the antenna of the radio terminal 1100A.

The radio wave received by the antenna of the radio terminal 1100Athrough the path P1 is a preceding wave (a direct wave). The radio wavesreceived by the antenna of the radio terminal 1100A through the paths P2and the P3 are delayed waves which are delayed from the preceding wave.

As shown in FIG. 2(b), the radio waves on the paths have mutuallydifferent delay time. In the example shown in FIG. 2(b), the antenna ofthe radio terminal 1100A receives the radio wave (the direct wave) onthe path P1 at delay time T 1, receives the radio wave (the reflectedwave) on the path P2 at delay time T 2, and receives the radio wave (thereflected wave) on the path P3 at delay time T 3. The antenna of theradio terminal 1100A receives these waves collectively as a compositewave.

In the OFDM scheme, the sender side adds a redundant signal sectioncalled a guard interval to each symbol in order to absorb a delay timedifference attributable to the above-described multiple paths.

FIG. 3(a) is a view showing a symbol structure in the OFDM scheme. Asshown in FIG. 3(a), a symbol in the OFDM scheme (hereinafter an OFDMsymbol) includes an effective symbol section in a finite time generatedby IFFT and the guard interval obtained by copying part of the effectivesymbol section.

By using the guard interval, if a time difference Tdmax (hereinafterreferred to as a “delay time difference”) between time of reception ofthe preceding wave and time of reception of the latest delayed wavefalls within a time length of the guard interval (hereinafter referredto as a “guard interval length”) Tg as shown in FIG. 3(b), the FFTfunctions normally on the receiver side whereby it is possible to avoidoccurrence of an intersymbol interference.

On the other hand, if there arises a delayed wave exceeding the guardinterval length Tg, an intersymbol interference occurs and the FFT doesnot function normally on the receiver side. Hence a large strain isgenerated and a communication performance is deteriorated.

Accordingly, when it is estimated that there arises the intersymbolinterference, the radio terminal 1100A according to the first embodimentswitches the used communication scheme from the OFDM scheme to the CDMAscheme.

(2) Configuration of Radio Terminal

Next, a configuration of the radio terminal 1100A will be described inthe order of (2. 1) Schematic Configuration, (2. 2) Configuration ofCommunication Unit, and (2. 3) Configuration of Control Unit.

(2. 1) Schematic Configuration

FIG. 4 is a block diagram showing a configuration of the radio terminal1100A. As shown in FIG. 4, the radio terminal 1100A includes an antenna1101, a communication unit 1120, measurement units 1140, a control unit1160A, and a storage unit 1180. Here, the number of antennas 1101 is notlimited only to one but may be plural.

The communication unit 1120 performs radio communication with the radiobase station 1200A or the radio base station 1300 through the antenna1101. The communication unit 1120 performs the radio communication withthe radio base station 1200A by using any of the CDMA scheme or the OFDMscheme.

The measurement unit 1140A measures reception quality of the compositewave including the preceding wave and the delayed waves received by thecommunication unit 1120.

The measurement unit 1140A measures a value (EVM (Error VectorMagnitude)) indicative of a difference between the OFDM symbol (namely,an effective symbol section) included in the composite wave received bythe communication unit 1120 and a reference point of the OFDM symbol asthe reception quality. Details of the EVM will be described later.

Meanwhile, the measurement unit 1140A measures a state of a voltagewaveform of the composite wave received by the communication unit 1120at a timing corresponding to each of the guard intervals.

The measurement unit 1140B measures reception quality of a radio signalcorresponding to the CDMA scheme, which is received by the communicationunit 1120.

The control unit 1160A is formed by use of a CPU, for example, and isconfigured to control various functions incorporated in the radioterminal 1100A. The control unit 1160A controls the communication unit1120 so as to switch the used communication scheme between the OFDMscheme and the CDMA scheme.

The storage unit 1180 is formed by use of a memory, for example, and isconfigured to store a variety of information used for the control by thecontrol unit 1160A.

(2. 2) Configuration of Communication Unit

Next, a configuration of the communication unit 1120 will be described.The communication unit 1120 includes a modulation unit 1121, a switchSW1, a CDMA transmission unit 1122A, an OFDM transmission unit 1122B, aswitch unit 1123, a CDMA reception unit 1124A, an OFDM reception unit1124B, and a demodulation unit 1125.

The modulation unit 1121 modulates and encodes transmission data to theradio base station 1200A. The modulation unit 1121 has a configurationsuitable for adaptive modulation. In the adaptive modulation, multiplemodulation schemes are predetermined based on a combination of amodulation multi-value number and an encoding ratio. The modulationschemes are also called modulation classes or MCS levels. The modulationunit 1121 modulates and encodes the transmission data in accordance witha certain modulation scheme selected from the multiple modulationschemes.

The switch SW1 inputs the transmission data outputted from themodulation unit 1121 to any of the CDMA transmission unit 1122A or theOFDM transmission unit 1122B in accordance with the control by thecontrol unit 1160A. The switch SW1 inputs the transmission data to theCDMA transmission unit 1122A when the used transmission scheme is theCDMA scheme, or inputs the transmission data to the OFDM transmissionunit 1122B when the used transmission scheme is the OFDM scheme.

The CDMA transmission unit 1122A subjects the inputted transmission datato spread spectrum in accordance with the CDMA scheme and to conversioninto a radio frequency band as well as to amplification processing,thereby generating a CDMA signal at the radio frequency band(hereinafter a CDMA wave). The generated CDMA wave is transmitted to theradio base station 1200A via the switch unit 1123 and the antenna 1101.

The OFDM transmission unit 1122B subjects the inputted transmission datato multi-carrier modulation in accordance with the OFDM scheme and toconversion into a radio frequency band as well as to amplificationprocessing, thereby generating an OFDM signal at the radio frequencyband (hereinafter an OFDM wave). The generated OFDM wave is transmittedto the radio base station 1200A via the switch unit 1123 and the antenna1101.

In accordance with the control by the control unit 1160A, the switchunit 1123 inputs the CDMA wave generated by the CDMA transmission unit1122A or the OFDM wave generated by the OFDM transmission unit 1122B tothe antenna 1101. The switch unit 1123 inputs the CDMA wave to theantenna 1101 when the used transmission scheme is the CDMA scheme, orinputs the OFDM wave to the antenna 1101 when the used transmissionscheme is the OFDM scheme.

On the other hand, at the time of reception, the switch unit 1123 inputsa radio signal received by the antenna 1101 to the CDMA reception unit1124A and the OFDM reception unit 1124B.

The CDMA reception unit 1124A subjects the inputted CDMA wave toconversion into a baseband and amplification processing, and performsinverse diffusion in accordance with the CDMA scheme. Moreover, the CDMAreception unit 1124A performs RAKE reception which is processing forcombining the preceding wave and the delayed waves included in the CDMAwave received from the radio base station 1200A. In the RAKE reception,the reception quality is improved by combining the preceding wave andthe delayed waves while aligning phases thereof. Reception data thusobtained are inputted to the demodulation unit 1125.

The OFDM reception unit 1124B subjects the inputted OFDM wave toconversion into the baseband and the amplification processing, andperforms multi-carrier demodulation in accordance with the OFDM scheme.Moreover, the OFDM reception unit 1124B removes the guard intervalsincluded in the OFDM wave. The reception data thus obtained are inputtedto the demodulation unit 1125.

The demodulation unit 1125 demodulates and decodes the inputtedreception data. The demodulation unit 1125 demodulates and decodes thereception data in accordance with a method corresponding to the certainmodulation scheme selected from the multiple modulation schemes.Moreover, the demodulation unit 1125 subjects the inputted receptiondata to symbol determination.

(2. 3) Configuration of Control Unit

Next, a configuration of the control unit 1160A will be described. Thecontrol unit 1160A includes a determination unit 1161, an acquisitionunit 1162, a switching control unit 1163, and a cell selection unit1164.

When the used communication scheme in an idle state is the OFDM scheme,the determination unit 1161 performs processing for determination(hereinafter determination processing) based on the EVM measured by themeasurement unit 1140A as to whether or not the delay time differenceTdmax exceeds the guard interval length Tg. The determination unit 1161determines whether or not the EVM measured by the measurement unit 1140Ais equal to or above an EVM threshold. In this embodiment, the EVM isreception quality related to a time difference between a preceding waveand a delayed wave of the received OFDM signal. In this embodiment, theEVM threshold is a threshold corresponding to the guard interval.

For example, the EVM threshold is preset to a value of the EVM at whichthe delay time difference Tdmax exceeds the guard interval length Tg.The value of the EVM at which the delay time difference Tdmax exceedsthe guard interval length Tg can be obtained on an experimental or anempirical basis.

On the basis of the modulation scheme used for the radio communication(namely, the downlink communication), the acquisition unit 1162 acquiresfrom the storage unit 1180 the EVM threshold to be used for thedetermination by the determination unit 1161. The acquisition unit 1162sets the acquired EVM threshold to the determination unit 1161.

The switching control unit 1163 controls the switch SW1 of thecommunication unit 1120 and the switch unit 1123. The switching controlunit 1163 controls the communication unit 1120 (namely, the switch SW1and the switch unit 1123) so as to switch the used communication schemefrom the OFDM scheme to the CDMA scheme.

The cell selection unit 1164 selects the cell representing acommunication area formed by the radio base station 1300 supporting theCDMA scheme. For example, if the reception quality (the EVM) from aradio base station supporting the OFDM scheme (a radio base stationlocated in the neighborhood) falls below a predetermined threshold (theEVM threshold), the cell selection unit 1164 reselects the cell to beformed by the radio base station 1300.

(3) Detailed Operations of Radio Terminal

Next, detailed operations of the radio terminal 1100A will be described.FIG. 5 is a flowchart showing operations of the radio terminal 1100Awhen the used communication scheme is the OFDM scheme. This operationflow is executed in a period when the radio terminal 1100A is in theidle state, or in a period when the radio terminal 1100A is standing bywithout communicating with a communication party except for the radiobase station, for example.

In step S1101, the measurement unit 1140A measures the state of thevoltage waveform of the OFDM wave received by the OFDM reception unit1124B.

The waveform measurement processing by the measurement unit 1140A isshown in FIG. 6. In the example of FIG. 6, the delay time difference isT as shown in FIG. 6(a), and the preceding wave shown in FIG. 6(b) iscombined with the delayed wave shown in FIG. 6(c) whereby the OFDM waveshown in FIG. 6(d) is received. The measurement unit 1140A specifies aguard interval period of the preceding wave in response to a result ofsymbol synchronization, for example, and measures the state of thevoltage wave form (a voltage value) of the OFDM wave at a measurementtiming corresponding to the guard interval period. This measurement isperformed at each measurement timing corresponding to each guardinterval period.

In step S1102, the measurement unit 1140A determines whether or not thestate of the voltage waveform measured at a current measurement timingT(n) is equal to a state of the voltage waveform measured at ameasurement timing (hereinafter referred to as a “precedent measurementtiming”) T(n−1) which is precedent to the current measurement timing.The process goes to step S1103 when the state of the voltage waveformmeasured at the current timing T(n) is equal to the state of the voltagewaveform measured at the precedent measurement timing T(n−1). In stepS1103, the value n is incremented and the process goes to subsequentmeasurement of the voltage waveform. On the other hand, the process goesto step S1104 if the state of the voltage waveform measured at thecurrent timing T(n) is different from the state of the voltage waveformmeasured at the precedent measurement timing T(n−1).

In step S1104, the measurement unit 1140A measures the EVM of the OFDMsymbol corresponding to the current measurement timing T(n). The EVM iscalled modulation accuracy, which represents an amount of deviation forthe phase and the amplitude of an observed symbol point S from a symbolreference point Sref where the symbol point is supposed to be located,as shown in FIG. 7(a). Specifically, the EVM is an effective value of anerror vector and is expressed as square root percentage of average powerof an ideal signal. FIG. 7(c) shows a calculation formula for the EVM.

In step S1105, the acquisition unit 1162 acquires the EVM threshold fromthe storage unit 1180. The storage unit 1180 stores a table which linksthe modulation schemes with the EVM thresholds as shown in FIG. 7(b). Amodulation scheme which can achieve communication at a higher speed (amodulation scheme involving a larger bit number per symbol) has stricterrestrictions on phase and amplitude errors. Accordingly, the EVMthreshold is set to a lower value as the modulation scheme can achievecommunication at a higher speed. Here, a modulation scheme (such asQPSK) at the slowest communication speed is used at initial connection.Therefore, an EVM threshold corresponding to that modulation scheme maybe used fixedly.

In step S1106, the determination unit 1161 determines whether or not theEVM measured in step S1104 is equal to or above the EVM thresholdacquired and set in step S1105. The process goes to step S1107 upondetermination that the EVM measured in step S1104 is equal to or abovethe EVM threshold acquired and set in step S1105. On the other hand, theprocess goes to step S1103 upon determination that the EVM measured instep S1104 is below the EVM threshold which is acquired and set in stepS1105.

In step S1107, the cell selection unit 1164 causes the measurement unit1140B to measure the reception quality of the radio signal correspondingto the CDMA scheme. When the result of measurement is a favorable resultin step S1108 (when a measured RSSI is equal to or above a giventhreshold, for example), the cell selection unit 1164 performsreselection of the cell to be formed by the radio base station 1300 instep S1109, and the switching control unit 1163 controls thecommunication unit 1120 (namely, the switch SW1 and the switch unit1123) in step S1110 so as to switch the used communication scheme fromthe OFDM scheme to the CDMA scheme. The communication unit 1120transmits a connection request to a CDMA-supporting radio base stationshowing a favorable result of measurement by the measurement unit 1140Bat the time of switching the used communication scheme from the OFDMscheme to the CDMA scheme. If there are multiple radio base stations totransmit the connection request, it is preferable to transmit therequest to the station showing the best measurement result. Meanwhile,step S1107 is executed when the measured EVM from each ofOFDM-supporting radio base stations located near the radio terminal1100A is equal to or above the EVM threshold. Steps S1107 to S1110 maybe the processing that complies with 3GPP TS36.300 standards and thelike.

(4) Operation and Effect

According to the first embodiment, the control unit 1160A checks thatthe reception quality of the reception quality of the CDMA scheme isfavorable under the situation where the delay time difference Tdmax isestimated to exceed the guard interval length Tg, and then controls thecommunication unit 1120 to switch the used communication scheme from theOFDM scheme to the CDMA scheme.

For this reason, it is possible to avoid an intersymbol interference inthe OFDM scheme and to prevent deterioration in a communicationperformance. Moreover, it is possible to utilize the excellentcommunication performance of the OFDM scheme until the delay timedifference Tdmax is estimated to exceed the guard interval length Tg.

Therefore, it is possible to provide a radio terminal which supportsboth of the CDMA scheme and the OFDM scheme and is capable of preventingdeterioration in the communication performance due to the intersymbolinterference while utilizing the OFDM scheme.

In the first embodiment, the cell selection unit 1164 causes themeasurement unit 1140B to measure the reception quality of the radiosignal of the CDMA scheme when the measured EVM is equal to or above theEVM threshold.

The EVM has advantageous features that it can be measured with a smalleramount of operation and that a shorter time is required for themeasurement as compared to other reception quality indices including aSNR (Signal to Noise ratio), a BER (Bit Error Rate), a channelestimation value, and the like.

For this reason, by using the EVM as the reception quality index, it ispossible to determine whether or not the delay time difference Tdmaxexceeds the guard interval length Tg easily and instantaneously.Therefore, it is possible to reduce a processing load and powerconsumption of the radio terminal 1100A as compared to the case of usingother reception quality indices, and to shorten a period ofdeterioration in the communication performance due to the intersymbolinterference.

In the first embodiment, when the communication unit 1120 performs theradio communication using the adaptive modulation, the acquisition unit1162 sets the EVM threshold based on the modulation scheme used for theradio communication. For this reason, it is possible to set the EVMthreshold appropriately even in the case of using the adaptivemodulation.

In the first embodiment, the measurement unit 1140A measures thereception quality (the EVM) only when the state of the voltage waveformmeasured at the current measurement timing T(n) is different from thestate of the voltage waveform measured at the precedent measurementtiming T(n−1).

When the state of the voltage waveform measured at the currentmeasurement timing T(n) is equal to the state of the voltage waveformmeasured at the precedent measurement timing T(n−1), it is possible toconsider that the multipath state remains unchanged. Accordingly, insuch a case, it is possible to reduce the processing load and the powerconsumption of the radio terminal 1100A by omitting the measurement ofthe EVM as well as the determination processing.

Further, in the case of performing the determination only by use of thereception quality (the EVM), there is a risk of erroneous determinationin the determination processing if the EVM varies due to a factor (suchas a circuit factor) other than a change in the multipath state.Accordingly, it is possible to improve determination accuracy bymeasuring the EVM only when the state of the voltage waveform measuredat the current measurement timing T(n) is different from the state ofthe voltage waveform measured at the precedent measurement timingT(n−1).

In the first embodiment, the CDMA reception unit 1124A performs the RAKEreception for combining the preceding wave and the delayed wavesincluded in the CDMA wave received from the radio base station 1200Awhen the used communication scheme is the CDMA scheme. Specifically, inthe CDMA scheme, a path diversity effect attributable to the RAKEreception is obtained when there is a large time difference between thepreceding wave and the delayed waves.

Therefore, it is possible to utilize the characteristics of the CDMAscheme and to effectively suppress deterioration in the communicationperformance by switching the used communication method from the OFDMscheme to the CDMA scheme.

Second Embodiment

A second embodiment is an embodiment configured to perform thedetermination processing on the radio base station side. In thefollowing, (1) Configuration of Radio Terminal, (2) Configuration ofRadio Base Station, (3) Operations of Radio Communication System, and(4) Operation and Effect will be described. It is to be noted, however,that only different features from those in the first embodiment will bedescribed below and duplicate explanation will be omitted.

(1) Configuration of Radio Terminal

FIG. 8 is a block diagram showing a configuration of a radio terminal1100B according to a second embodiment. As shown in FIG. 8, a controlunit 1160B of the radio terminal 1100B does not include thedetermination unit 1161 and the acquisition unit 1162 as described inthe first embodiment. Instead, the control unit 1160B includes anotification processing unit 1165 configured to exchange a variety ofnotification with a radio base station 1200B (see FIG. 9) according tothe second embodiment. The notification processing unit 1165 notifiesreception quality information indicative of the EVM measured by themeasurement unit 1140 to the radio base station 1200B by use of thecommunication unit 1120.

(2) Configuration of Radio Base Station

FIG. 9 is a block diagram showing a configuration of a radio basestation 1200B according to the second embodiment. As shown in FIG. 9,the radio base station 1200B includes an antenna 1201, a communicationunit 1220, a control unit 1240, and a storage unit 1260.

The communication unit 1220 performs radio communication with the radioterminal 1100B through the antenna 1201. The Control unit 1240 is formedby use of a CPU, for example, and is configured to control variousfunctions incorporated in the radio base station 1200B. The storage unit1260 is formed by use of a memory, for example, and is configured tostore a variety of information used for the control by the control unit1240.

The communication unit 1220 receives from the radio terminal 1100B thereception quality information indicative of the EVM measured by theradio terminal 1100B.

The control unit 1240 includes an acquisition unit 1241, a determinationunit 1242, and an indication unit 1243.

The determination unit 1242 determines whether or not the EVM indicatedin the reception quality information received by the communication unit1220 is equal to or above the EVM threshold.

On the basis of the modulation scheme used for the radio communication(namely, the downlink communication), the acquisition unit 1241 acquiresfrom the storage unit 1260 the threshold to be used for thedetermination by the determination unit 1242. The acquisition unit 1241sets the acquired threshold to the determination unit 1242.

The instruction unit 1243 generates a measurement instruction forinstructing the measurement of the reception quality of the receptionquality of the OFDM scheme when the determination unit 1242 determinesthat the EVM is equal to or above the EVM threshold.

The measurement instruction generated by the instruction unit 1243 istransmitted to the radio terminal 1100B by the communication unit 1220.

(3) Operations of Radio Communication System

FIG. 10 is a sequence chart showing operations of the radio terminal1100B and the radio base station 1200B.

In step S1201, the communication unit 1220 of the radio base station1200B transmits the OFDM wave to the radio terminal 1100B. The OFDMreception unit 1124B of the radio terminal 1100B receives the OFDM wave.

In step S1202, the measurement unit 1140 of the radio terminal 1100Bmeasures the state of the voltage waveform of the OFDM wave received bythe OFDM reception unit 1124B at a measurement timing corresponding toeach of the guard intervals. The measurement unit 1140 measures the EVMonly when the state of the voltage waveform measured at the currentmeasurement timing (T(n)) is different from the state of the voltagewaveform measured at the measurement timing (T(n−1)) precedent to thecurrent measurement timing.

In step S1203, the notification processing unit 1165 of the radioterminal 1100B notifies the reception quality information indicative ofthe EVM measured by the measurement unit 1140 to the radio base station1200B by using the OFDM transmission unit 1122B of the communicationunit 1120.

In step S1204, the determination unit 1242 of the radio base station1200B determines whether or not the EVM indicated in the receptionquality information received by the communication unit 1220 is equal toor above the EVM threshold. Here, an assumption is made that the EVM isdetermined to be equal to or above the EVM threshold.

In step S1205, the communication unit 1220 of the radio base station1200B transmits the measurement instruction generated by the instructionunit 1243 to the radio terminal 1100B. The OFDM reception unit 1124B ofthe radio terminal 1100B receives the measurement instruction. Themeasurement instruction received by the OFDM reception unit 1124B isforwarded to the notification processing unit 1165.

In step S1206, the cell selection unit 1164 of the radio terminal 1100Bcauses the measurement unit 1140B to measure the reception quality ofthe reception signal of the CDMA scheme in accordance with themeasurement instruction forwarded to the notification processing unit1165.

(4) Operation and Effect

According to the second embodiment, by performing the determinationprocessing on the radio base station 1200B side, it is possible toreduce the processing load and the power consumption of the radioterminal 1100B as compared to the first embodiment.

Modified Examples of First Embodiment and Second Embodiment

The first embodiment and the second embodiment have described theexample of individually providing the CDMA communication unit (the CDMAtransmission unit 1122A and the CDMA reception unit 1124A) and the OFDMcommunication unit (the OFDM transmission unit 1122B and the OFDMreception unit 1124B). However, a configuration may be made so that theCDMA communication unit and the OFDM communication unit are formedcollectively as one communication unit. For example, in a radio terminalcalled a cognitive terminal, the communication scheme can be switched byway of software by downloading software (SDR BB, Tunable RF)corresponding to the used communication scheme.

Meanwhile, in order to achieve reduction in the power consumption, theswitching control unit 1163 may be configured to stop power supply tothe communication unit of the CDMA communication unit (the CDMAtransmission unit 1122A and the CDMA reception unit 1124A) and the OFDMcommunication unit (the OFDM transmission unit 1122A and the OFDMreception unit 1124B), which is not in the used communication scheme.

In the first embodiment and the second embodiment, the EVM is used asthe reception quality index. However, another reception quality index(such as the SNR, the BER or the channel estimation value) may be used.

In the first embodiment and the second embodiment, the LTE has beendescribed as the example of the radio communication system employing theOFDM scheme. However, without limitation to the LTE, it is also possibleto use UMB (Ultra Mobile Broadband) standardized by 3GPP2, WiMAXstandardized by IEEE 802.16, a next-generation PHS, and the like.

Third Embodiment

Next, a third embodiment of the present invention will be described.Specifically, (1) Outline of Radio Communication System, (2)Configuration of Radio Terminal, (3) Configuration of Radio BaseStation, (4) Operations of Radio Communication System, (5) Effect ofThird Embodiment, and (6) Modified Example of Third Embodiment will bedescribed.

(1) Outline of Radio Communication System

FIG. 11 is an overall schematic diagram showing a radio communicationsystem 2010 according to a third embodiment.

As shown in FIG. 11, the radio communication system 2010 includes aradio terminal 2100, a radio base station 2200 (a first radio basestation), and a radio base station 2300 (a second radio base station).

The radio terminal 2100 is a dual terminal supporting both of a CDMAscheme and an OFDM scheme. The radio base station 2200 supports the OFDMscheme, while the radio base station 2300 supports the CDMA scheme. Notethat an OFDMA (Orthogonal Frequency Division Multiplexing Access) schemeis assumed to be included in the OFDM scheme in this embodiment.

In the third embodiment, the communication standard of the OFDM schemeis the LTE (Long Term Evolution) standardized by the 3GPP (3rdGeneration Partnership Project). In the LTE, the OFDM scheme is used forthe downlink. In the following, the downlink will be mainly described.Meanwhile, in the third embodiment, the communication standard in theCDMA scheme is cdma2000 standardized by 3GPP2. In the cdma2000, the CDMAscheme is used for both of the uplink (a reverse link) and the downlink(a forward link).

The radio base station 2200 constitutes part of a LTE network (called“E-UTRAN”) 2020. The radio base station 2300 constitutes part of acdma2000 network 2030. The LTE network 2020 includes a MME (MobilityManagement Entity) 2025 which is a management device configured tomanage mobility of the radio terminal 2100.

The radio terminal 2100 is in a connected state of being connected tothe radio base station 2200 (hereinafter referred to as an “activescheme”). Specifically, the radio terminal 2100 is connected to theradio base station 2200 and is performing communication with acommunication destination device (such as a server or a communicationterminal) via the radio base station 2200. The radio terminal 2100 islocated in a communicatable area of the radio base station 2200 and isalso located in a communicatable area of the radio base station 2300.

The OFDM scheme is the scheme configured to distribute data to multiplesubcarriers that are orthogonal to one another and to modulate thesubcarriers. A sender side generates the OFDM signal by subjecting eachof the subcarriers either to multi-phase PSK modulation or tomulti-value QAM modulation and then by subjecting each of thesubcarriers to inverse fast Fourier transform (IFFT). A receiver sideperforms demodulation by subjecting the OFDM signal to fast Fouriertransform (FFT).

When the radio terminal 2100 and the radio base station 2200 performradio communication in an environment unable to see each other directly,for example, an antenna 2101 (see FIG. 14) of the radio terminal 2100receives multiple radio waves (multipath waves) taking different pathsas shown in FIG. 12(a). In the example of FIG. 12(a), a path P1 directlyreaching the antenna 2201 of the radio terminal 2100 and paths P2 and P3reaching the antenna 2101 of the radio terminal 2100 after reflection bya building or the ground are formed between an antenna 2201 (see FIG.15) of the radio base station 2200 and the antenna 2101 of the radioterminal 2100.

The radio wave received by the antenna 2101 of the radio terminal 2100through the path P1 is the preceding wave (the direct wave). The radiowaves received by the antenna 2101 of the radio terminal 2100 throughthe paths P2 and the P3 are the delayed waves which are delayed from thepreceding wave.

As shown in FIG. 12(b), the radio waves on the respective paths havemutually different delay time. In the example shown in FIG. 12(b), theantenna 2101 of the radio terminal 2100 receives the radio wave (thedirect wave) on the path P1 at delay time T 1, receives the radio wave(the reflected wave) on the path P2 at delay time T 2, and receives theradio wave (the reflected wave) on the path P3 at delay time T 3. Theantenna 2101 of the radio terminal 2100 receives these wavescollectively as the composite wave.

In the OFDM scheme, the sender side adds the redundant signal sectioncalled the guard interval to each symbol in order to absorb the delaytime difference attributable to the above-described multiple paths.

FIG. 13(a) is a view showing a symbol structure in the OFDM scheme. Asshown in FIG. 13(a), the symbol in the OFDM scheme (hereinafter the OFDMsymbol) includes the effective symbol section in a finite time generatedby the IFFT and the guard interval obtained by copying part of theeffective symbol section.

By using the guard interval, if the time difference Tdmax (hereinafterreferred to as the “delay time difference”) between time of reception ofthe preceding wave and time of reception of the latest delayed wavefalls within the time length of the guard interval Tg as shown in FIG.13(b), the FFT functions normally on the receiver side whereby it ispossible to avoid occurrence of an intersymbol interference.

On the other hand, if there arises a delayed wave exceeding the guardinterval length Tg, an intersymbol interference occurs and the FFT doesnot function normally on the receiver side. Hence a large strain isgenerated and a communication performance is deteriorated. Accordingly,when it is estimated that there arises the intersymbol interference atthe radio terminal 2100, the radio base station 2200 according to thethird embodiment causes the radio terminal 2100 to execute a handoverfrom the radio base station 2200 to the radio base station 2300.

(2) Configuration of Radio Terminal

FIG. 14 is a block diagram showing a configuration of the radio terminal2100.

As shown in FIG. 14, the radio terminal 2100 includes an antenna 2101, amodulation unit 2121, a transmission unit 2122 (terminal transmissionunit), a duplexer 2123, a reception unit 2124 (terminal reception unit),a demodulation unit 2125, an OFDM measurement unit 2141 (a firstmeasurement unit), a CDMA measurement unit 2142 (a second measurementunit), a control unit 2160, and a storage unit 2180. The transmissionunit 2122 includes a switch SW1, a CDMA transmission unit 2122A, and anOFDM transmission unit 2122B. The reception unit 2124 includes a switchSW2, a CDMA reception unit 2124A, and an OFDM reception unit 2124B.

The modulation unit 2121 modulates and encodes transmission data fromthe control unit 2160. The modulation unit 2121 has a configurationsuitable for adaptive modulation. In the adaptive modulation, multiplemodulation schemes are predetermined based on a combination of amodulation multi-value number and an encoding ratio. The modulationschemes are also called modulation classes or MCS (Modulation and CodingScheme) levels. The modulation unit 2121 modulates and encodes thetransmission data in accordance with a certain modulation schemeselected from the multiple modulation schemes.

The switch SW1 inputs the transmission data outputted from themodulation unit 2121 to any of the CDMA transmission unit 2122A or theOFDM transmission unit 2122B in accordance with the control by thecontrol unit 2160. The switch SW1 inputs the transmission data to theCDMA transmission unit 2122A when the used transmission scheme is theCDMA scheme, or inputs the transmission data to the OFDM transmissionunit 2122B when the used transmission scheme is the OFDM scheme.

The CDMA transmission unit 2122A subjects the inputted transmission datato spread spectrum in accordance with the CDMA scheme and to conversioninto a radio frequency band as well as to amplification processing,thereby generating a CDMA signal at the radio frequency band. Thegenerated CDMA wave is transmitted via the duplexer 2123 and the antenna2101.

The OFDM transmission unit 2122B subjects the inputted transmission datato multi-carrier modulation in accordance with the OFDM scheme and toconversion into a radio frequency band as well as to amplificationprocessing, thereby generating an OFDM signal at the radio frequencyband. The generated OFDM signal is transmitted via the duplexer 2123 andthe antenna 2101.

The duplexer 2123 inputs the radio signal (either the CDMA signal or theOFDM signal) to the antenna 2101. Meanwhile, at the time of reception,the duplexer 2123 inputs the radio signal (either the CDMA signal or theOFDM signal) received by the antenna 2101 to the switch SW2.

The switch SW2 inputs the radio signal from the duplexer 2123 to any ofthe CDMA reception unit 2124A or the OFDM reception unit 2124B inaccordance with the control by the control unit 2160. The switch SW2inputs the radio signal from the duplexer 2123 to the CDMA receptionunit 2124A when the used transmission scheme is the CDMA scheme, orinputs the radio signal from the duplexer 2123 to the OFDM receptionunit 2124B when the used transmission scheme is the OFDM scheme.

The CDMA reception unit 2124A subjects the inputted radio signal (theCDMA signal) to conversion into the baseband and the amplificationprocessing, and performs inverse diffusion in accordance with the CDMAscheme. Moreover, the CDMA reception unit 2124A performs the RAKEreception which is the processing for combining the preceding wave andthe delayed waves included in the received CDMA signal. In the RAKEreception, the reception quality is improved by combining the precedingwave and the delayed waves while aligning the phases thereof. Thereception data thus obtained are inputted to the demodulation unit 2125.

The OFDM reception unit 2124B subjects the inputted radio signal (theOFDM signal) to conversion into the baseband and the amplificationprocessing, and performs multi-carrier demodulation in accordance withthe OFDM scheme. Moreover, the OFDM reception unit 2124B removes theguard intervals included in the received OFDM signal. The reception dataobtained as a consequence are inputted to the demodulation unit 2125.

The demodulation unit 2125 demodulates and decodes the inputtedreception data. The demodulation unit 2125 demodulates and decodes thereception data in accordance with a method corresponding to the certainmodulation scheme selected from the multiple modulation schemes.Moreover, the demodulation unit 2125 subjects the inputted receptiondata to the symbol determination.

The OFDM measurement unit 2141 measures reception parameters whichindicate the time difference between the preceding wave and the delayedwaves of the received OFDM signal. In the third embodiment, thereception parameters include an amplitude error and a phase errorbetween the OFDM symbol S included in the received OFDM signal and thereference point Sref of the OFDM symbol as shown in FIG. 16(a). Thevalues of the reception parameters (the amplitude error and the phaseerror) grow larger as the delay time difference Tdmax exceeds the guardinterval Tg by a larger degree. The reception parameters measured by theOFDM measurement unit 2141 are inputted to the control unit 2160.

The CDMA measurement unit 2142 measures the reception quality of theCDMA signal. In the third embodiment, the received signal strengthindicator (RSSI) is used as the reception quality of the CDMA signal.However, without limitation to the RSSI, it is also possible to use thereceived SNR (Signal to Noise ratio) and the like. The RSSI measured bythe CDMA measurement unit 2142 is inputted to the control unit 2160.

The control unit 2160 is formed by use of a CPU, for example, and isconfigured to control various functions incorporated in the radioterminal 2100. The storage unit 2180 is formed by use of a memory, forexample, and is configured to store a variety of information used forthe control by the control unit 2160.

The control unit 2160 controls the switches SW1, 2. The control unit2160 switches between the switches SW1, 2 when switching the usedcommunication scheme between the OFDM scheme and the CDMA scheme.Moreover, the control unit 2160 may temporarily switches between theswitches SW1, 2, when measuring the RSSI of the CDMA signal during theOFDM communication.

The control unit 2160 inputs the reception parameters measured by theOFDM measurement unit 2141 and the RSSI measured by the OFDM measurementunit 2141 to the modulation unit 2121. The OFDM transmission unit 2122Btransmits the reception parameters and the RSSI after the modulation tothe radio base station 2200. Such a report on the measurement results iscalled a “Measurement Report”.

The control unit 2160 operates the OFDM measurement unit 2141 and theCDMA measurement unit 2142 in response to the instruction from the radiobase station 2200, and executes the handover for switching from theradio base station 2200 to the radio base station 2300.

(3) Configuration of Radio Base Station

FIG. 15 is a block diagram showing a configuration of the radio basestation 2200.

As shown in FIG. 15, the radio base station 2200 includes an antenna2201, a modulation unit 2221, a transmission unit 2222 (base-stationtransmission unit), a duplexer 2223, a reception unit 2224 (base-stationreception unit), a demodulation unit 2225, a control unit 2240, astorage unit 2260, and a wired communication unit 2280.

The modulation unit 2221 modulates and encodes the transmission datafrom the control unit 2240. The modulation unit 2221 modulates andencodes the transmission data based on the certain modulation schemeselected from the multiple modulation schemes in accordance with theadaptive modulation.

The transmission unit 2222 subjects the inputted transmission data tomulti-carrier modulation in accordance with the OFDM scheme and toconversion into a radio frequency band as well as to amplificationprocessing, thereby generating an OFDM signal at the radio frequencyband. The generated OFDM signal is transmitted via the duplexer 2223 andthe antenna 2201.

The duplexer 2223 inputs the OFDM signal to the antenna 2201. Meanwhile,at the time of reception, the duplexer 2223 inputs the OFDM signalreceived by the antenna 2201 to the reception unit 2224.

The reception unit 2224 subjects the inputted OFDM signal to conversioninto the baseband and the amplification processing, and performsmulti-carrier demodulation in accordance with the OFDM scheme. Moreover,the reception unit 2224 removes the guard intervals included in the OFDMsignal. The reception data thus obtained are inputted to thedemodulation unit 2225.

The demodulation unit 2225 demodulates and decodes the inputtedreception data. The demodulation unit 2225 demodulates and decodes thereception data in accordance with a method corresponding to the certainmodulation scheme selected from the multiple modulation schemes.Moreover, the demodulation unit 2225 subjects the inputted receptiondata to the symbol determination.

The control unit 2240 is formed by use of a CPU, for example, and isconfigured to control various functions incorporated in the radio basestation 2200. The storage unit 2260 is formed by use of a memory, forexample, and is configured to store a variety of information used forthe control by the control unit 2240. The wired communication unit 2280performs communication with the LTE network 2020 side.

The control unit 2240 transmits the measurement instruction for the RSSIof the CDMA signal to the radio terminal 2100 by using the transmissionunit 2222. In the third embodiment, the transmission unit 2222 transmitsthe measurement instruction either periodically or upon occurrence of apredetermined trigger.

The storage unit 2260 stores a neighbor list in advance, which includesinformation on radio base stations (hereinafter nearby base stations)located near the radio base station 2200. In the third embodiment, theneighbor list includes IDs, used channel information, and the like ofthe radio base stations (such as the radio base station 2300) supportingthe CDMA scheme. The control unit 2240 incorporates the neighbor list inthe measurement instruction and transmits the information from thetransmission unit 2222.

The control unit 2240 acquires the reception parameters and the RSSItransmitted from the radio terminal 2100 in response to the measurementinstruction via the reception unit 2224 and the demodulation unit 2225.The control unit 2240 determines whether or not to cause the radioterminal 2100 to perform the handover based on the reception parametersand the RSSI.

Specifically, the control unit 2240 calculates the EVM (Error VectorMagnitude) by use of the reception parameters and compares thecalculated EVM with the EVM threshold. The EVM threshold is determinedbased on the guard interval used in the OFDM scheme and is stored inadvance in the storage unit 2260.

The EVM threshold is preset to the value of the EVM when the delay timedifference Tdmax exceeds the guard interval length Tg. The value of theEVM at which the delay time difference Tdmax exceeds the guard intervallength Tg can be obtained on an experimental or an empirical basis.

In the third embodiment, the EVM threshold is provided for each of themodulation schemes to be used in the adaptive modulation. As shown inFIG. 16(a), the EVM is also referred to as the modulation accuracy,which represents an effective value of an error vector based on theamount of deviation (reception parameter) for the phase and theamplitude of the observed symbol point S from the symbol reference pointSref where the symbol point is supposed to be located and is expressedas square root percentage of average power of an ideal signal. FIG.16(c) shows a calculation formula for the EVM.

The storage unit 2260 stores a table which links the modulation schemeswith the EVM thresholds as shown in FIG. 16(b). A modulation schemewhich can achieve communication at a higher speed (a modulation schemeinvolving a larger bit number per symbol) has stricter restrictions onphase and amplitude errors. Accordingly, the EVM threshold is set to alower value as the modulation scheme can achieve communication at ahigher speed.

The control unit 2240 acquires the EVM threshold from the storage unit2260 based on the modulation scheme used for the downlink and comparesthe EVM threshold with the calculated EVM.

Moreover, the control unit 2240 compares the RSSI of the CDMA signalwith a predetermined value. When the RSSI is higher than thepredetermined value, it is possible to assume that the reception qualityis favorable. The predetermined value is preset to the value of the RSSIwith which the radio terminal 2100 can execute the communication.

Regarding the control unit 2240, when the calculated EVM exceeds the EVMthreshold and the RSSI is higher than the predetermined value, thecontrol unit 2240 of the radio base station 2200 decides to cause theradio terminal 2100 to execute the handover to the radio base station(such as the radio base station 2300) supporting the CDMA scheme. Whencausing the radio terminal 2100 to carry out the handover, the controlunit 2240 transmits an instruction for the handover to the radioterminal 2100 by use of the transmission unit 2222 after checking thatthe handover is feasible.

(4) Operations of Radio Communication System

Next, operations of the radio communication system 2010 will bedescribed by using (4. 1) Operation Pattern 1 and (4. 2) OperationPattern 2 as examples. In the third embodiment, the operations of theradio communication system 2010 are based on the 3GPP TS36.300 standardsand the like. The operation pattern 1 is an operation pattern configuredto cause the radio terminal 2100 to measure the RSSI of the CDMA signalperiodically. The operation pattern 2 is an operation pattern configuredto cause the radio terminal 2100 to measure the RSSI of the CDMA signalby using a certain event as a trigger.

(4. 1) Operation Pattern 1

FIG. 17 is a sequence chart showing an operation pattern 1 of the radiocommunication system 2010. This sequence is executed when the radioterminal 2100 is in an active scheme.

In step S2101, the transmission unit 2222 of the radio base station 2200transmits the measurement instruction to the radio terminal 2100. Theabove-described neighbor list is included in the measurementinstruction. The OFDM reception unit 2124B of the radio terminal 2100receives the measurement instruction.

In step S2102, the control unit 2160 of the radio terminal 2100 causesthe CDMA measurement unit 2142 to measure the RSSI of the CDMA signalfor each of the CDMA-supporting base stations corresponding to the IDsincluded in the neighbor list.

In step S2103, the control unit 2160 of the radio terminal 2100 causesthe OFDM measurement unit 2141 to measure the reception parameters ofthe OFDM signal received from the radio base station 2200.

In step S2104, the OFDM transmission unit 2122B of the radio terminal2100 transmits a measurement result report including the receptionparameters measured by the OFDM measurement unit 2141 and the RSSImeasured by the CDMA measurement unit 2142 to the radio base station2200. The reception unit 2224 of the radio base station 2200 receivesthe measurement result report (the reception parameters and the RSSI).

In step S2105, the control unit 2240 of the radio base station 2200calculates the EVM by use of the reception parameters which are receivedby the reception unit 2224 and demodulated by the demodulation unit2225.

In step S2106, the control unit 2240 of the radio base station 2200compares the calculated EVM with the EVM threshold corresponding to themodulation scheme. Meanwhile, the control unit 2240 of the radio basestation 2200 compares the RSSI, which is received by the reception unit2224 and demodulated by the demodulation unit 2225, with thepredetermined value.

When the calculated EVM exceeds the EVM threshold and the RSSI is higherthan the predetermined value, the control unit 2240 of the radio basestation 2200 decides to cause the radio terminal 2100 to execute thehandover to the CDMA-supporting radio base station (step S2107). On theother hand, when the calculated EVM falls below the EVM threshold orwhen the RSSI is equal to or below the predetermined value, the controlunit 2240 of the radio base station 2200 decides not to cause the radioterminal 2100 to execute the handover to the CDMA-supporting radio basestation.

Here, when there are multiple CDMA-supporting radio base stations havingthe RSSI higher than the predetermined value in the case where thecalculated EVM exceeds the EVM threshold, it is preferable that thecontrol unit 2240 of the radio base station 2200 determine theCDMA-supporting radio base station having the highest RSSI as a handoverdestination. In the following, a case of causing the radio terminal 2100to execute the handover to the radio base station 2300 will bedescribed.

In step S2108, the transmission unit 2222 of the radio base station 2200transmits a handover preparation instruction to the radio terminal 2100.The OFDM reception unit 2124B of the radio terminal 2100 receives thehandover preparation instruction.

In step S2109, the OFDM transmission unit 2122B of the radio terminal2100 transmits a request for connection to the radio base station 2300to the radio base station 2200. This request for connection is forwardedto the radio base station 2300 by tunneling under the supervision of theMME 2025 (step S2110). When the request for connection is successful,the MME 2025 notifies the radio base station 2200 of the fact (stepS2111).

In step S2112, the radio base station 2200 transmits an instruction forthe handover to the radio base station 2300 to the radio terminal 2100in response to the notification from the MME 2025. Upon receipt of theinstruction for the handover to the radio base station 2300, the radioterminal 2100 executes the handover to the radio base station 2300.

(4. 2) Operation Pattern 2

FIG. 18 is a sequence chart showing an operation pattern 2 of the radiocommunication system 2010. This sequence is executed when the radioterminal 2100 is in an active scheme.

In step S2201, the control unit 2160 of the radio terminal 2100 causesthe OFDM measurement unit 2141 to measure the reception parameters ofthe OFDM signal received from the radio base station 2200.

In step S2202, the OFDM transmission unit 2122B of the radio terminal2100 transmits the reception parameters measured by the OFDM measurementunit 2141 to the radio base station 2200. The reception unit 2224 of theradio base station 2200 receives the reception parameters.

In step S2203, the control unit 2240 of the radio base station 2200calculates the EVM by use of the reception parameters which are receivedby the reception unit 2224 and demodulated by the demodulation unit2225.

In step S2204, the control unit 2240 of the radio base station 2200compares the calculated EVM with the EVM threshold corresponding to themodulation scheme.

When the calculated EVM exceeds the EVM threshold, the transmission unit2222 of the radio base station 2200 transmits a RSSI measurementinstruction to the radio terminal 2100 in step S2205. Theabove-described neighbor list is included in the measurementinstruction. The OFDM reception unit 2124B of the radio terminal 2100receives the measurement instruction.

In step S2206, the control unit 2160 of the radio terminal 2100 causesthe CDMA measurement unit 2142 to measure the RSSI of the CDMA signalfor each of the CDMA-supporting base stations corresponding to the IDsincluded in the neighbor list.

In step S2207, the OFDM transmission unit 2122B of the radio terminal2100 transmits the measurement result report including the RSSI measuredby the CDMA measurement unit 2142 to the radio base station 2200. Thereception unit 2224 of the radio base station 2200 receives themeasurement result report (the RSSI).

In step S2208, the control unit 2240 of the radio base station 2200compares the RSSI, which is received by the reception unit 2224 anddemodulated by the demodulation unit 2225, with the predetermined value.

When the RSSI is higher than the predetermined value, the control unit2240 of the radio base station 2200 decides to cause the radio terminal2100 to execute the handover to the CDMA-supporting radio base station(step S2209). On the other hand, when the RSSI is equal to or below thepredetermined value, the control unit 2240 of the radio base station2200 decides not to cause the radio terminal 2100 to execute thehandover to the CDMA-supporting radio base station. Here, when there aremultiple CDMA-supporting radio base stations having the RSSI higher thanthe predetermined value, it is preferable that the control unit 2240 ofthe radio base station 2200 determine the CDMA-supporting radio basestation having the highest RSSI as the handover destination.

Each processing in steps S2210 to S2214 is executed as similar to theoperation pattern 1.

(5) Effect of Third Embodiment

According to the third embodiment, the control unit 2240 of the radiobase station 2200 decides to cause the radio terminal 2100 to executethe handover to the radio base station 2300 when the EVM exceeds the EVMthreshold and the RSSI is higher than the predetermined value. Here, theEVM reflects the delay time difference Tdmax between the preceding waveand the delayed waves of the OFDM signal and the EVM threshold is set tothe value of the EVM when the delay time difference Tdmax exceeds theguard interval length Tg. The state of the EVM exceeding the EVMthreshold means that the delay time difference Tdmax exceeds the guardinterval length Tg.

Accordingly, under the situation where the delay time difference Tdmaxis estimated to exceed the guard interval length Tg, it is possible toavoid an intersymbol interference and to prevent deterioration in thecommunication performance by causing the radio terminal 2100 to executethe handover to the radio base station 2300 supporting the CDMA schemeafter checking that a reception level of the CDMA signal is favorable.Moreover, it is possible to utilize the excellent communicationperformance of the OFDM scheme until the delay time difference Tdmax isestimated to exceed the guard interval length Tg.

Therefore, the radio terminal 2100 supporting both of the CDMA schemeand the OFDM scheme can prevent deterioration in the communicationperformance due to the intersymbol interference while utilizing the OFDMscheme in the active scheme.

Furthermore, the CDMA reception unit 2124A of the radio terminal 2100can perform the RAKE reception in which the preceding wave and thedelayed waves in the CDMA signal are combined. Accordingly, it ispossible to achieve high resistance to the multi-path environment and toobtain a path diversity effect by the RAKE reception. Therefore, it ispossible to utilize the characteristic of the CDMA scheme and toeffectively suppress deterioration in the communication performance byswitching the used communication scheme from the OFDM scheme to the CDMAscheme.

Meanwhile, the EVM (and reception parameter) has the advantageousfeatures that it can be measured with a smaller amount of operation andthat a shorter time is required for the measurement as compared to otherreception quality indices including the SNR (Signal to Noise ratio), theBER (Bit Error Rate), the channel estimation value, and the like. Forthis reason, by using the EVM, it is possible to estimate whether or notthe delay time difference Tdmax exceeds the guard interval length Tgeasily and instantaneously. Therefore, it is possible to reduce aprocessing load and power consumption of the radio terminal 2100 ascompared to the case of using other reception quality indices, and toshorten a period of deterioration in the communication performance dueto the intersymbol interference.

In the third embodiment, the control unit 2240 sets the EVM thresholdcorresponding to the modulation scheme used for the downlink radiocommunication. Accordingly, it is possible to set the EVM thresholdvalue appropriately even when using the adaptive modulation.

(6) Modified Examples of Third Embodiment

It is also possible to measure the reception parameters of the OFDMsignal only when the state of the voltage waveform of the received OFDMsignal satisfies a certain condition. FIG. 19 shows waveform measurementprocessing by the OFDM measurement unit 2141. In the example of FIG. 19,the delay time difference is equal to T as shown in FIG. 19(a), and thepreceding wave shown in FIG. 19(b) is combined with the delayed waveshown in FIG. 19(c), and the OFDM signal shown in FIG. 19(d) isreceived. The OFDM measurement unit 2141 specifies the guard intervalperiod of the preceding wave based on a result of symbolsynchronization, for example, and measures the state of the voltagewaveform (a voltage value) of the OFDM signal at measurement timingcorresponding to the guard interval period. The measurement is performedat each measurement timing corresponding to each of the guard intervalperiods. The OFDM measurement unit 2141 determines whether or not thestate of the voltage waveform measured at the current measurement timingT(n) is equal to the state of the voltage waveform measured at themeasurement timing precedent to the current measurement timing(hereinafter referred to as the “precedent measurement timing”) T(n−1).The OFDM measurement unit 2141 omits the measurement of the receptionparameters when the state of the voltage waveform measured at thecurrent measurement timing T(n) is equal to the state of the voltagewaveform measured at the precedent measurement timing T(n−1), orexecutes the measurement of the reception parameters when the state ofthe voltage waveform measured at the current measurement timing T(n) isdifferent from the state of the voltage waveform measured at theprecedent measurement timing T(n−1). In the case of performing thedetermination only by use of the EVM, there is a risk of erroneousdetermination if the EVM varies due to a factor (such as a circuitfactor) other than a change in the multipath state. Accordingly, it ispossible to improve determination accuracy by measuring the EVM onlywhen the state of the voltage waveform measured at the currentmeasurement timing T(n) is different from the state of the voltagewaveform measured at the precedent measurement timing T(n−1).

The third embodiment has described the example of individually providingthe CDMA communication unit (the CDMA transmission unit 2122A and theCDMA reception unit 2124A) and the OFDM communication unit (the OFDMtransmission unit 2122B and the OFDM reception unit 2124B). However, itis also possible to use an aspect of forming the CDMA communication unitand the OFDM communication unit collectively as one communication unit.For example, in a radio terminal called a cognitive terminal, it ispossible to switch the communication scheme by way of software bydownloading software (SDR BB, Tunable RF) corresponding to the usedcommunication scheme.

In the third embodiment, the reception parameters to be measured by theradio terminal 2100 include the amplitude error and the phase error andthe EVM is calculated by the radio base station 2200. However, the radioterminal 2100 may calculate the EVM by use of the amplitude error andthe phase error and transmit the EVM as the reception parameter to theradio base station 2200. In this case, the “value corresponding to thereception parameter” is the value of the EVM. Alternatively, withoutlimitation to the case of using the EVM, it is also possible to useanother reception quality index (such as the SNR, the BER or the channelestimation value).

In the third embodiment, the cellular phone terminal is shown as theexample of the radio terminal 2100. However, without limitation to thecellular phone terminal, it is possible to use a terminal equipped withcommunication devices according to the CDMA scheme and the OFDM scheme,for example.

Although the third embodiment has described the example in which theguard interval length is a fixed length, the guard interval length mayalso be a variable length. For example, in the case of selectively usingtwo types of guard intervals, namely, a short guard interval and a longguard interval longer than the short guard interval, it is preferable tomeasure the reception parameters (or the EVM) at the time of the longguard interval from the viewpoint of ensuring the measurement time.

In the third embodiment, the LTE has been described as the example ofthe radio communication system employing the OFDM scheme. However,without limitation to the LTE, it is also possible to use WiMAXstandardized by IEEE 802.16, a next-generation PHS (XGP), and the like.

Fourth Embodiment

Next, a fourth embodiment of the present invention will be described.Specifically, (1) Outline of Radio Communication System, (2)Configuration of Radio Terminal, (3) Configuration of Radio BaseStation, (4) Operations of Radio Communication System, (5) Effect ofFourth Embodiment, and (6) Modified Example of Fourth Embodiment will bedescribed.

(1) Outline of Radio Communication System

FIG. 20 is an overall schematic diagram showing a radio communicationsystem 3010 according to a fourth embodiment.

As shown in FIG. 20, the radio communication system 3010 includes aradio terminal 3100, a radio base station 3200, and a radio base station3300.

In the fourth embodiment, the radio terminal 3100 is a dual terminalsupporting both of a CDMA scheme and a next-generation communicationscheme (a certain communication scheme). The radio base station 3200supports the next-generation communication scheme, while the radio basestation 3300 supports the CDMA scheme. Here, the next-generationcommunication scheme is any of the OFDM scheme and the SC-FDMA scheme.Note that an OFDMA (Orthogonal Frequency Division Multiplexing Access)scheme is assumed to be included in the OFDM scheme in this embodiment.

In the fourth embodiment, the communication standard of thenext-generation communication scheme is the LTE (Long Term Evolution)standardized by the 3GPP (3rd Generation Partnership Project). In theLTE, the OFDM scheme is used for the downlink, while the SC-FDMA schemeis used for the uplink. In the following, the uplink will be mainlydescribed.

In the fourth embodiment, the communication standard in the CDMA schemeis cdma2000 standardized by 3GPP2. In the cdma2000, the CDMA scheme isused for both of the uplink (a reverse link) and the downlink (a forwardlink).

The radio base station 3200 constitutes part of a LTE network (called“E-UTRAN”) 3020. The radio base station 3300 constitutes part of acdma2000 network 2030. The LTE network 3020 includes a MME (MobilityManagement Entity) 3025 which is a management device configured tomanage mobility of the radio terminal 3100.

The radio terminal 3100 is in a connected state of being connected tothe radio base station 3200 (hereinafter referred to as an “activescheme”). Specifically, the radio terminal 3100 is connected to theradio base station 3200 and is performing communication with acommunication destination device (such as a server or a communicationterminal) via the radio base station 3200. The radio terminal 3100 islocated in a communicatable area of the radio base station 3200 and isalso located in a communicatable area of the radio base station 3300.

The OFDM scheme is the scheme configured to distribute data to multiplesubcarriers that are orthogonal to one another and to modulate thesubcarriers. A sender side generates the OFDM signal by subjecting eachof the subcarriers either to multi-phase PSK modulation or tomulti-value QAM modulation and then by subjecting each of thesubcarriers to inverse fast Fourier transform (IFFT). A receiver sideperforms demodulation by subjecting the OFDM signal to fast Fouriertransform (FFT). The OFDM scheme is not suitable for the uplink due to ahigh peak-to-average power ratio (PAPR). Accordingly, the SC-FDMA schemeis used for the uplink of the LTE as the technique that can reduce thePAPR. In the SC-FDMA scheme, a sender side generates a SC-FDMA signal byperforming the IFFT on the transmission signal after being subjected todiscrete Fourier transform (DFT).

When the radio terminal 3100 and the radio base station 3200 performradio communication in an environment unable to see each other directly,for example, an antenna 3201 (see FIG. 24) of the radio base station3200 receives multiple radio waves (multipath waves) taking differentpaths as shown in FIG. 21(a). In the example of FIG. 21(a), a path P1directly reaching the antenna 3201 of the radio base station 3200 andpaths P2 and P3 reaching the antenna 3201 of the radio base station 3200after reflection by a building or the ground are formed between anantenna 3101 (see FIG. 23) of the radio terminal 3100 and the antenna3201 of the radio base station 3200.

The radio wave received by the antenna 3201 of the radio base station3200 through the path P1 is the preceding wave (the direct wave). Theradio waves received by the antenna 3201 of the radio base station 3200through the paths P2 and the P3 are the delayed waves which are delayedfrom the preceding wave.

As shown in FIG. 21(b), the radio waves on the respective paths havemutually different delay time. In the example shown in FIG. 21(b), theantenna 3201 of the radio base station 3200 receives the radio wave (thedirect wave) on the path P1 at delay time T 1, receives the radio wave(the reflected wave) on the path P2 at delay time T 2, and receives theradio wave (the reflected wave) on the path P3 at delay time T 3. Theantenna 3201 of the radio base station 3200 receives these wavescollectively as the composite wave.

In the OFDM scheme and the SC-FDMA scheme, the sender side adds theredundant signal section called the guard interval to each symbol inorder to absorb the delay time difference attributable to theabove-described multiple paths.

FIG. 22(a) is a view showing a symbol structure in the SC-FDMA scheme.As shown in FIG. 22(a), the symbol in the SC-FDMA scheme (hereinafterthe SC-FDMA symbol) includes an effective symbol section and a guardinterval obtained by copying part of the effective symbol section.

By using the guard interval, if the time difference Tdmax (hereinafterreferred to as the “delay time difference”) between time of reception ofthe preceding wave and time of reception of the latest delayed wavefalls within the time length of the guard interval Tg as shown in FIG.22(b), the FFT functions normally on the receiver side whereby it ispossible to avoid occurrence of an intersymbol interference.

On the other hand, if there arises a delayed wave exceeding the guardinterval length Tg, an intersymbol interference occurs and thedemodulation is not performed normally on the receiver side. Hence alarge strain is generated and a communication performance isdeteriorated. Accordingly, when it is estimated that there arises theintersymbol interference in the uplink, the radio base station 3200according to the fourth embodiment causes the radio terminal 3100 toexecute a handover to the radio base station 3300 supporting CDMAscheme.

(2) Configuration of Radio Terminal

FIG. 23 is a block diagram showing a configuration of the radio terminal3100.

As shown in FIG. 23, the radio terminal 3100 includes an antenna 3101, amodulation unit 3121, a transmission unit 3122, a duplexer 3123, areception unit 3124, a demodulation unit 3125, a measurement unit 3150,a control unit 3160, and a storage unit 3180. The transmission unit 3122includes a switch SW1, a CDMA transmission unit 3122, and an SC-FDMAtransmission unit 3122B. The reception unit 3124 includes a switch SW2,a CDMA reception unit 3124A, and an OFDM reception unit 3124B.

The modulation unit 3121 modulates and encodes transmission data fromthe control unit 3160. The modulation unit 3121 has a configurationsuitable for adaptive modulation. In the adaptive modulation, multiplemodulation schemes are predetermined based on a combination of amodulation multi-value number and an encoding ratio. The modulationschemes are also called modulation classes or MCS (Modulation and CodingScheme) levels. The modulation unit 3121 modulates and encodes thetransmission data in accordance with a certain modulation schemeselected from the multiple modulation schemes.

The switch SW1 inputs the transmission data outputted from themodulation unit 3121 to any of the CDMA transmission unit 3122A or theSC-FDMA transmission unit 3122B in accordance with the control by thecontrol unit 3160. The switch SW1 inputs the transmission data to theCDMA transmission unit 3122A when the used transmission scheme is theCDMA scheme, or inputs the transmission data to the SC-FDMA transmissionunit 3122B when the used transmission scheme is the SC-FDMA scheme.

The CDMA transmission unit 3122A subjects the inputted transmission datato spread spectrum in accordance with the CDMA scheme and to conversioninto a radio frequency band as well as to amplification processing,thereby generating a CDMA signal at the radio frequency band. Thegenerated CDMA wave is transmitted via the duplexer 3123 and the antenna3101.

The SC-FDMA transmission unit 3122B generates an SC-FDMA signal from theinputted transmission data, and performs conversion into a radiofrequency band as well as amplification processing, thereby generatingan SC-FDMA signal at the radio frequency band. The generated SC-FDMAsignal is transmitted via the duplexer 3123 and the antenna 3101.

In the meantime, the duplexer 3123 inputs the radio signal (the CDMAsignal or the OFDM signal) received by the antenna 3101 to the switchSW2 at the time of reception.

The switch SW2 inputs a radio signal from the duplexer 3123 to any ofthe CDMA reception unit 3124A or the OFDM reception unit 3124B inaccordance with the control by the control unit 3160. The switch SW2inputs the radio signal from the duplexer 3123 to the CDMA receptionunit 3124A when the used transmission scheme is the CDMA scheme, orinputs a radio signal from the duplexer 3123 to the OFDM reception unit3124B when the used transmission scheme is the OFDM scheme.

The CDMA reception unit 3124A subjects the inputted radio signal (theCDMA signal) to conversion into the baseband and the amplificationprocessing, and performs inverse diffusion in accordance with the CDMAscheme. Moreover, the CDMA reception unit 3124A performs the RAKEreception which is the processing of combining the preceding wave andthe delayed waves included in the received CDMA signal. In the RAKEreception, the reception quality is improved by combining the precedingwave and the delayed waves while aligning the phases thereof. Thereception data thus obtained are inputted to the demodulation unit 3125.

The OFDM reception unit 3124B subjects the inputted radio signal (theOFDM signal) to conversion into the baseband and the amplificationprocessing, and performs multi-carrier demodulation in accordance withthe OFDM scheme. Moreover, the OFDM reception unit 3124B removes theguard intervals included in the received OFDM signal. The reception datathus obtained are inputted to the demodulation unit 3125.

The demodulation unit 3125 demodulates and decodes the inputtedreception data. The demodulation unit 3125 demodulates and decodes thereception data in accordance with a method corresponding to the certainmodulation scheme selected from the multiple modulation schemes.Moreover, the demodulation unit 3125 subjects the inputted receptiondata to the symbol determination.

The CDMA measurement unit 3150 measures the reception quality of theCDMA signal. In the fourth embodiment, the received signal strengthindicator (RSSI) is used as the reception quality of the CDMA signal.However, without limitation to the RSSI, it is also possible to use thereceived SNR (Signal to Noise ratio) and the like. The RSSI measured bythe CDMA measurement unit 3150 is inputted to the control unit 3160.

The control unit 3160 is formed by use of a CPU, for example, and isconfigured to control various functions incorporated in the radioterminal 3100. The storage unit 3180 is formed by use of a memory, forexample, and is configured to store a variety of information used forthe control by the control unit 3160.

The control unit 3160 controls the switches SW1, 2. The control unit3160 switches between the switches SW1, 2 when switching the usedcommunication scheme between the OFDM scheme and a next-generationcommunication scheme (SC-FDMA scheme, OFDM scheme). Moreover, thecontrol unit 3160 may temporarily switches between the switches SW1, 2,when measuring the RSSI of the CDMA signal during the next-generationcommunication.

The control unit 3160 inputs the RSSI (hereinafter measured RSSI)measured by the measurement unit 3150 to the modulation unit 3221. TheSC-FDMA transmission unit 3122B transmits the measured RSSI after themodulation to the radio base station 3200. Such a report on themeasurement results is called a “Measurement Report”.

The control unit 3160 operates the measurement unit 3150 in response tothe instruction from the radio base station 3200, and executes thehandover for switching from the radio base station 3200 to the radiobase station 3300.

(3) Configuration of Radio Base Station

FIG. 24 is a block diagram showing a configuration of the radio basestation 3200.

As shown in FIG. 24, the radio base station 3200 includes an antenna3201, a modulation unit 3221, a transmission unit 3222, a duplexer 3223,a reception unit 3224, a demodulation unit 3225, a control unit 3240, ameasurement unit 3250, a storage unit 3260, and a wired communicationunit 3280.

The modulation unit 3221 modulates and encodes the transmission datafrom the control unit 3240. The modulation unit 3221 modulates andencodes the transmission data based on the certain modulation schemeselected from the multiple modulation schemes in accordance with theadaptive modulation.

The CDMA transmission unit 3122 subjects the inputted transmission datato multi-carrier modulation in accordance with the OFDM scheme and toconversion into a radio frequency band as well as to amplificationprocessing, thereby generating an OFDM signal at the radio frequencyband. The generated OFDM signal is transmitted via the duplexer 3123 andthe antenna 3201.

Meanwhile, at the time of reception, the duplexer 3223 inputs theSC-FDMA signal received by the antenna 3201 to the reception unit 3224.The reception unit 3224 subjects the inputted SC-FDMA signal toconversion into the baseband and the amplification processing, andperforms demodulation in accordance with the SC-FDMA scheme. Moreover,the reception unit 3224 removes the guard intervals included in thereceived SC-FDMA signal. The reception data thus obtained are inputtedto the demodulation unit 3225.

The demodulation unit 3225 demodulates and decodes the inputtedreception data. The demodulation unit 3225 demodulates and decodes thereception data in accordance with a method corresponding to the certainmodulation scheme selected from the multiple modulation schemes.Moreover, the demodulation unit 3225 subjects the inputted receptiondata to the symbol determination.

The measurement unit 3250 measures a reception parameter which indicatesa time difference between the preceding wave and the delayed waves ofthe received SC-FDMA signal. In the fourth embodiment, the receptionparameter is the EVM (Error Vector Magnitude) which is calculated by useof an amplitude error and a phase error between a SC-FDMA symbol Sincluded in the received SC-FDMA signal and a reference point Sref ofthe SC-FDMA symbol as shown in FIG. 25(a). The value of the EVM growslarger as the delay time difference Tdmax exceeds the guard interval Tgby a larger degree. The EVM measured by the measurement unit 3250 isinputted to the control unit 3240.

The control unit 3240 is formed by use of a CPU, for example, and isconfigured to control various functions incorporated in the radio basestation 3200. The storage unit 3260 is formed by use of a memory, forexample, and is configured to store a variety of information used forthe control by the control unit 3240. The wired communication unit 3280performs communication with the LTE network 3020 side.

The control unit 3240 transmits the measurement instruction for the RSSIof the CDMA signal to the radio terminal 3100 by using the transmissionunit 3222. In the fourth embodiment, the transmission unit 3222transmits the measurement instruction either periodically or uponoccurrence of a predetermined trigger.

The storage unit 3260 stores the neighbor list in advance, whichincludes the information on the radio base stations located near theradio base station 3200. In the fourth embodiment, the neighbor listincludes the IDs, the used channel information, and the like of theradio base stations (such as the radio base station 3300) supporting theCDMA scheme. The control unit 3240 incorporates the neighbor list in themeasurement instruction and transmits the information from thetransmission unit 3222.

The control unit 3240 acquires the RSSI measured by the radio terminal3100 in response to the measurement instruction via the reception unit3224 and the demodulation unit 3225. The control unit 3240 determineswhether or not to cause the radio terminal 3100 to perform the handoverbased on the acquired measured RSSI.

Specifically, the control unit 3240 compares the measured EVM with theEVM threshold. The EVM threshold is determined based on the guardinterval used in the SC-FDMA scheme and is stored in advance in thestorage unit 3260. The EVM threshold is preset to the value of the EVMwhen the delay time difference Tdmax exceeds the guard interval Tg. Thevalue of the EVM at which the delay time difference Tdmax exceeds theguard interval Tg can be obtained on an experimental or an empiricalbasis.

In the fourth embodiment, the EVM threshold is provided for each of themodulation schemes to be used in the adaptive modulation. As shown inFIG. 25(a), the EVM is also referred to as the modulation accuracy,which represents an effective value of an error vector based on theamount of deviation (EVM) for the phase and the amplitude of theobserved symbol point S from the symbol reference point Sref where thesymbol point is supposed to be located and is expressed as square rootpercentage of average power of an ideal signal. FIG. 25(c) shows acalculation formula for the EVM.

The storage unit 3260 stores a table which links the modulation schemeswith the EVM thresholds as shown in FIG. 25(b). A modulation schemewhich can achieve communication at a higher speed (a modulation schemeinvolving a larger bit number per symbol) has stricter restrictions onphase and amplitude errors. Accordingly, the EVM threshold is set to alower value as the modulation scheme can achieve communication at ahigher speed.

The control unit 3240 acquires the EVM threshold from the storage unit3260 based on the modulation scheme used for the uplink and compares theEVM threshold with the calculated EVM.

Moreover, the control unit 3240 compares the measured RSSI of the CDMAsignal with a predetermined value so as to determine whether or not theCDMA reception quality is favorable. When the measured RSSI is higherthan the predetermined value, it is possible to assume that thereception quality is favorable. The predetermined value is preset to thevalue of the RSSI with which the radio terminal 3100 can execute thecommunication.

When the calculated EVM exceeds the EVM threshold and the RSSI is higherthan the predetermined value, the control unit 3240 of the radio basestation 3200 decides to cause the radio terminal 3100 to execute thehandover to the radio base station (such as the radio base station 3300)supporting the CDMA scheme. When causing the radio terminal 3100 tocarry out the handover, the control unit 3240 transmits an instructionfor the handover to the radio terminal 3100 by use of the transmissionunit 3222 after checking that the handover is feasible.

(4) Operations of Radio Communication System

Next, operations of the radio communication system 3010 will bedescribed by using (4. 1) Operation Pattern 1 and (4. 2) OperationPattern 2 as examples. In the fourth embodiment, the operations of theradio communication system 3010 are based on the 3GPP TS36.300 standardsand the like. The operation pattern 1 is an operation pattern configuredto cause the radio terminal 3100 to measure the RSSI of the CDMA signalperiodically. The operation pattern 2 is an operation pattern configuredto cause the radio terminal 3100 to measure the RSSI of the CDMA signalby using a certain event as a trigger.

(4. 1) Operation Pattern 1

FIG. 26 is a sequence chart showing an operation pattern 1 of the radiocommunication system 3010. This sequence is executed when the radioterminal 3100 is in an active scheme.

In step S3101, the transmission unit 3222 of the radio base station 3200transmits the measurement instruction to the radio terminal 3100. Theabove-described neighbor list is included in the measurementinstruction. The OFDM reception unit 3124B of the radio terminal 3100receives the measurement instruction.

In step S3102, the control unit 3160 of the radio terminal 3100 causesthe measurement unit 3150 to measure the RSSI of the CDMA signal foreach of the CDMA-supporting base stations corresponding to the IDsincluded in the neighbor list.

In step S3103, the SC-FDMA transmission unit 3122B of the radio terminal3100 transmits a measurement result report including the RSSI measuredby the measurement unit 3150 to the radio base station 3200. Thereception unit 3224 of the radio base station 3200 receives themeasurement result report (the measured RSSI).

In step S3104, the measurement unit 3150 of the radio base station 3200measures the EVM of the SC-FDMA signal received from the radio terminal3100 by the reception unit 3224.

In step S3105, the control unit 3240 of the radio base station 3200compares the EVM measured by the measurement unit 3150 with the EVMthreshold corresponding to the modulation scheme. Meanwhile, the controlunit 3240 of the radio base station 3200 compares the measured RSSI,which is received by the reception unit 3224 and demodulated by thedemodulation unit 3225, with the predetermined value.

When the measured EVM exceeds the EVM threshold and the measured RSSI ishigher than the predetermined value, the control unit 3240 of the radiobase station 3200 decides to cause the radio terminal 3100 to executethe handover to the CDMA-supporting radio base station (step S3106). Onthe other hand, when the measured EVM falls below the EVM threshold orwhen the measured RSSI is equal to or below the predetermined value, thecontrol unit 3240 of the radio base station 3200 decides not to causethe radio terminal 3100 to execute the handover to the CDMA-supportingradio base station.

Here, when there are multiple CDMA-supporting radio base stations havingthe measured RSSI higher than the predetermined value in the case wherethe measured EVM exceeds the EVM threshold, it is preferable that thecontrol unit 3240 of the radio base station 3200 determine theCDMA-supporting radio base station having the highest measured RSSI as ahandover destination. In the following, a case of causing the radioterminal 3100 to execute the handover to the radio base station 3300will be described.

In step S3107, the transmission unit 3222 of the radio base station 3200transmits a handover preparation instruction to the radio terminal 3100.The OFDM reception unit 3124B of the radio terminal 3100 receives thehandover preparation instruction.

In step S3108, the SC-FDMA transmission unit 3122B of the radio terminal3100 transmits a request for connection to the radio base station 3300to the radio base station 3200. This request for connection is forwardedto the radio base station 3300 by tunneling under the supervision of theMME 3025 (step S3109). When the request for connection is successful,the MME 3025 notifies the radio base station 3200 of the fact (stepS3110).

In step S3111, the radio base station 3200 transmits an instruction forthe handover to the radio base station 3300 to the radio terminal 3100in response to the notification from the MME 3025. Upon receipt of theinstruction for the handover to the radio base station 3300, the radioterminal 3100 executes the handover to the radio base station 3300.

(4. 2) Operation Pattern 2

FIG. 27 is a sequence chart showing an operation pattern 2 of the radiocommunication system 3010. This sequence is executed when the radioterminal 3100 is in an active scheme.

In step S3201, the control unit 3240 of the radio base station 320causes the measurement unit 3250 to measure the SC-FDMA signal receivedfrom the radio terminal 3100 by the reception unit 3224.

In step S3202, the control unit 3240 of the radio base station 3200compares the EVM measured by the measurement unit 3250 with the EVMthreshold corresponding to the modulation scheme of the uplink.

When the measured EVM exceeds the EVM threshold, the control unit 3240of the radio base station 3200 causes the transmission unit 3222 totransmit a RSSI measurement instruction to the radio terminal 3100 instep S3203. The above-described neighbor list is included in themeasurement instruction. The OFDM reception unit 3124B of the radioterminal 3100 receives the measurement instruction.

In step S3204, the control unit 3160 of the radio terminal 3100 causesthe measurement unit 3150 to measure the RSSI of the CDMA signal foreach of the CDMA-supporting base stations corresponding to the IDsincluded in the neighbor list.

In step S3205, the SC-FDMA transmission unit 3122B of the radio terminal3100 transmits the measurement result report including the RSSI measuredby the measurement unit 3150 to the radio base station 3200. Thereception unit 3224 of the radio base station 3200 receives themeasurement result report (the measured RSSI).

In step S3206, the control unit 3240 of the radio base station 3200compares the measured RSSI, which is received by the reception unit 3224and demodulated by the demodulation unit 3225, with the predeterminedvalue.

When the measured RSSI is higher than the predetermined value, thecontrol unit 3240 of the radio base station 3200 decides to cause theradio terminal 3100 to execute the handover to the CDMA-supporting radiobase station (step S3207). On the other hand, when the measured RSSI isequal to or below the predetermined value, the control unit 3240 of theradio base station 3200 decides not to cause the radio terminal 3100 toexecute the handover to the CDMA-supporting radio base station. Here,when there are multiple CDMA-supporting radio base stations having themeasured RSSI higher than the predetermined value, it is preferable thatthe control unit 3240 of the radio base station 3200 determine theCDMA-supporting radio base station having the highest measured RSSI asthe handover destination.

Each processing in steps S3208 to S3212 is executed as similar to theoperation pattern 1.

(5) Effect of Fourth Embodiment

According to the fourth embodiment, the control unit 3240 of the radiobase station 3200 decides to cause the radio terminal 3100 to executethe handover to the radio base station 3300 when the EVM of the SC-FDMAsignal exceeds the EVM threshold and the measured RSSI is higher thanthe predetermined value. Here, the EVM reflects the delay timedifference Tdmax between the preceding wave and the delayed waves of theSC-FDMA signal and the EVM threshold is set to the value of the EVM whenthe delay time difference Tdmax exceeds the guard interval Tg. The stateof the measured EVM exceeding the EVM threshold means that the delaytime difference Tdmax exceeds the guard interval length Tg.

Accordingly, under the situation where the delay time difference Tdmaxof the SC-FDMA signal is estimated to exceed the guard interval lengthTg, it is possible to avoid an intersymbol interference and to preventdeterioration in the communication performance by causing the radioterminal 3100 to execute the handover to the radio base station 3300supporting the CDMA scheme after checking that a reception level of theCDMA signal is favorable. Moreover, it is possible to utilize theexcellent communication performance of the next-generation communicationscheme until the delay time difference Tdmax is estimated to exceed theguard interval Tg.

Therefore, the radio terminal 3100 supporting both of the CDMA schemeand the next-generation communication scheme can prevent deteriorationin the communication performance due to the intersymbol interferencewhile utilizing the next-generation communication scheme in the activescheme.

Furthermore, the CDMA reception unit 3124A of the radio terminal 3100can perform the RAKE reception in which the preceding wave and thedelayed waves in the CDMA signal are combined. Accordingly, it ispossible to achieve high resistance to the multi-path environment and toobtain a path diversity effect by the RAKE reception. Therefore, it ispossible to utilize the characteristic of the CDMA scheme and toeffectively suppress deterioration in the communication performance byswitching the used communication scheme from the next-generationcommunication scheme to the CDMA scheme.

Meanwhile, the EVM has the advantageous features that it can be measuredwith a smaller amount of operation and that a shorter time is requiredfor the measurement as compared to other reception quality indicesincluding the SNR (Signal to Noise ratio), the BER (Bit Error Rate), thechannel estimation value, and the like. For this reason, by using theEVM, it is possible to estimate whether or not the delay time differenceTdmax exceeds the guard interval length Tg easily and instantaneously.

In the fourth embodiment, the control unit 3240 sets the EVM thresholdcorresponding to the modulation scheme used for the uplink radiocommunication. Accordingly, it is possible to set the EVM thresholdvalue appropriately even when using the adaptive modulation in theuplink.

(6) Modified Examples of Fourth Embodiment

It is also possible to measure the EVM of the SC-FDMA signal only whenthe state of the voltage waveform of the received SC-FDMA signalsatisfies a certain condition. FIG. 28 shows waveform measurementprocessing by the measurement unit 3250. In the example of FIG. 28, thedelay time difference is equal to T as shown in FIG. 28(a), and thepreceding wave shown in FIG. 28(b) is combined with the delayed waveshown in FIG. 28(c), and the SC-FDMA signal shown in FIG. 28(d) isreceived. The measurement unit 3250 specifies the guard interval periodof the preceding wave based on a result of symbol synchronization, forexample, and measures the state of the voltage waveform (a voltagevalue) of the SC-FDMA signal at measurement timing corresponding to theguard interval period. The measurement is performed at each measurementtiming corresponding to each of the guard interval periods. Themeasurement unit 3250 determines whether or not the state of the voltagewaveform measured at the current measurement timing T(n) is equal to thestate of the voltage waveform measured at the measurement timingprecedent to the current measurement timing (hereinafter referred to asthe “precedent measurement timing”) T(n−1). The measurement unit 3250omits the measurement of the EVM when the state of the voltage waveformmeasured at the current measurement timing T(n) is equal to the state ofthe voltage waveform measured at the precedent measurement timingT(n−1), or executes the measurement of the EVM when the state of thevoltage waveform measured at the current measurement timing T(n) isdifferent from the state of the voltage waveform measured at theprecedent measurement timing T(n−1). In the case of performing thedetermination only by use of the EVM, there is a risk of erroneousdetermination if the EVM varies due to a factor (such as a circuitfactor) other than a change in the multipath state. Accordingly, it ispossible to improve determination accuracy by measuring the EVM onlywhen the state of the voltage waveform measured at the currentmeasurement timing T(n) is different from the state of the voltagewaveform measured at the precedent measurement timing T(n−1).

The fourth embodiment has described the example of individuallyproviding the CDMA communication unit (the CDMA transmission unit 3122Aand the CDMA reception unit 3124A) and the communication unit of thenext-generation communication scheme (the SC-FDMA transmission unit3122B and the OFDM reception unit 3124B). However, it is also possibleto use an aspect of forming these communication units collectively asone communication unit. For example, in a radio terminal called acognitive terminal, it is possible to switch the communication scheme byway of software by downloading software (SDR BB, Tunable RF)corresponding to the used communication scheme.

Although the fourth embodiment has described the example in which theguard interval length is a fixed length, the guard interval length mayalso be a variable length. For example, in the case of selectively usingtwo types of guard intervals, namely, a short guard interval and a longguard interval longer than the short guard interval, it is preferable tomeasure the EVM at the time of the long guard interval from theviewpoint of ensuring the measurement time.

In the fourth embodiment, the reception parameter to be measured by themeasurement unit 3250 is the EVM. However, without limitation to theEVM, it is also possible to use another reception quality index (such asthe SNR, the BER or the channel estimation value).

In the fourth embodiment, the cellular phone terminal is shown as theexample of the radio terminal 3100. However, without limitation to thecellular phone terminal, it is possible to use a terminal equipped withcommunication devices according to the CDMA scheme and thenext-generation communication scheme, for example.

In the fourth embodiment, the LTE has been described as the example ofthe radio communication system employing the OFDM scheme. However,without limitation to the LTE, it is also possible to use WiMAXstandardized by IEEE 802.16, a next-generation PHS (XGP), and the like.In the LTE, the SC-FDMA scheme is used for the uplink, while in theWiMAX or the like, the OFDM scheme is used for the uplink.

Fifth Embodiment

Next, a fifth embodiment of the present invention will be described.Specifically, (1) Outline of Radio Communication System, (2)Configuration of Radio Terminal, (3) Configuration of Radio BaseStation, (4) Operations of Radio Communication System, (5) Effect ofFifth Embodiment, and (6) Modified Example of Fifth Embodiment will bedescribed.

(1) Outline of Radio Communication System

FIG. 29 is an overall schematic diagram showing a radio communicationsystem 4010 according to a fifth embodiment.

As shown in FIG. 29, the radio communication system 4010 includes aradio terminal 4100, a radio base station 4200 (a first radio basestation), and a radio base station 4300 (a second radio base station).

The radio terminal 4100 is a dual terminal which supports both of theCDMA scheme and the OFDM scheme. The radio base station 4200 supportsthe OFDM scheme while the radio base station 4300 supports the CDMAscheme. Note that the OFDMA (Orthogonal Frequency Division MultiplexingAccess) scheme is assumed to be included in the OFDM scheme in thisembodiment.

In the fifth embodiment, the communication standard of the OFDM schemeis the LTE (Long Term Evolution) standardized by the 3GPP (3rdGeneration Partnership Project). In the LTE, the OFDM scheme is used forthe downlink. In the following, the downlink will be mainly described.Meanwhile, in the fifth embodiment, the communication standard in theCDMA scheme is cdma2000 standardized by 3GPP2. In the cdma2000, the CDMAscheme is used for both of the uplink (a reverse link) and the downlink(a forward link).

The radio base station 4200 constitutes part of a LTE network (called“E-UTRAN”) 4020. The radio base station 4300 constitutes part of acdma2000 network 4030. The LTE network 4020 includes a MME (MobilityManagement Entity) 4025 which is a management device configured tomanage mobility of the radio terminal 4100.

The radio terminal 4100 is in a connected state of being connected tothe radio base station 4200 (hereinafter referred to as an “activescheme”). Specifically, the radio terminal 4100 is connected to theradio base station 4200 and is performing communication with acommunication destination device (such as a server or a communicationterminal) via the radio base station 4200. The radio terminal 4100 islocated in a communicatable area of the radio base station 4200 and isalso located in a communicatable area of the radio base station 4300.

The OFDM scheme is the scheme configured to distribute data to multiplesubcarriers that are orthogonal to one another and to modulate thesubcarriers. A sender side generates the OFDM signal by subjecting eachof the subcarriers either to multi-phase PSK modulation or tomulti-value QAM modulation and then by subjecting each of thesubcarriers to inverse fast Fourier transform (IFFT). A receiver sideperforms demodulation by subjecting the OFDM signal to fast Fouriertransform (FFT).

When the radio terminal 4100 and the radio base station 4200 performradio communication in an environment unable to see each other directly,for example, an antenna 4101 (see FIG. 32) of the radio terminal 4100receives multiple radio waves (multipath waves) taking different pathsas shown in FIG. 30(a). In the example of FIG. 30(a), a path P1 directlyreaching the antenna 4201 of the radio terminal 4100 and paths P2 and P3reaching the antenna 4101 of the radio terminal 4100 after reflection bya building or the ground are formed between an antenna 4201 (see FIG.33) of the radio base station 4200 and the antenna 4101 of the radioterminal 4100.

The radio wave received by the antenna 4101 of the radio terminal 4100through the path P1 is the preceding wave (the direct wave). The radiowaves received by the antenna 4101 of the radio terminal 4100 throughthe paths P2 and the P3 are the delayed waves which are delayed from thepreceding wave.

As shown in FIG. 30(b), the radio waves on the respective paths havemutually different delay time. In the example shown in FIG. 30(b), theantenna 4101 of the radio terminal 4100 receives the radio wave (thedirect wave) on the path P1 at delay time T 1, receives the radio wave(the reflected wave) on the path P2 at delay time T 2, and receives theradio wave (the reflected wave) on the path P3 at delay time T 3. Theantenna 4101 of the radio terminal 4100 receives these wavescollectively as the composite wave.

In the OFDM scheme, the sender side adds the redundant signal sectioncalled the guard interval to each symbol in order to absorb the delaytime difference attributable to the above-described multiple paths.

FIG. 31(a) is a view showing a symbol structure in the OFDM scheme. Asshown in FIG. 31(a), the symbol in the OFDM scheme (hereinafter the OFDMsymbol) includes the effective symbol section in a finite time generatedby the IFFT and the guard interval obtained by copying part of theeffective symbol section.

By using the guard interval, if the time difference Tdmax (hereinafterreferred to as the “delay time difference”) between time of reception ofthe preceding wave and time of reception of the latest delayed wavefalls within the time length of the guard interval Tg as shown in FIG.31(b), the FFT functions normally on the receiver side whereby it ispossible to avoid occurrence of an intersymbol interference.

On the other hand, if there arises a delayed wave exceeding the guardinterval length Tg, an intersymbol interference occurs and the FFT doesnot function normally on the receiver side. Hence a large strain isgenerated and a communication performance is deteriorated. Accordingly,when it is estimated that there arises the intersymbol interference atthe radio terminal 4100, the radio base station 4200 according to thefifth embodiment causes the radio terminal 4100 to execute a handoverfrom the radio base station 4200 to the radio base station 4300.

(2) Configuration of Radio Terminal

FIG. 32 is a block diagram showing a configuration of the radio terminal4100.

As shown in FIG. 32, the radio terminal 4100 includes an antenna 4101, amodulation unit 4121, a transmission unit 4122 (terminal transmissionunit), a duplexer 4123, a reception unit 4124 (terminal reception unit),a demodulation unit 4125, an OFDM measurement unit 4141 (a firstmeasurement unit), a CDMA measurement unit 4142 (a second measurementunit), a control unit 4160, and a storage unit 4180. The transmissionunit 4122 includes a switch SW1, a CDMA transmission unit 4122A, and anOFDM transmission unit 4122B. The reception unit 4124 includes a switchSW2, a CDMA reception unit 4124A, and an OFDM reception unit 4124B.

The modulation unit 4121 modulates and encodes transmission data fromthe control unit 4160. The modulation unit 4121 has a configurationsuitable for adaptive modulation. In the adaptive modulation, multiplemodulation schemes are predetermined based on a combination of amodulation multi-value number and an encoding ratio. The modulationschemes are also called modulation classes or MCS (Modulation and CodingScheme) levels. The modulation unit 4121 modulates and encodes thetransmission data in accordance with a certain modulation schemeselected from the multiple modulation schemes.

The switch SW1 inputs the transmission data outputted from themodulation unit 4121 to any of the CDMA transmission unit 4122A or theOFDM transmission unit 4122B in accordance with the control by thecontrol unit 4160. The switch SW1 inputs the transmission data to theCDMA transmission unit 4122A when the used transmission scheme is theCDMA scheme, or inputs the transmission data to the OFDM transmissionunit 4122B when the used transmission scheme is the OFDM scheme.

The CDMA transmission unit 4122A subjects the inputted transmission datato spread spectrum in accordance with the CDMA scheme and to conversioninto a radio frequency band as well as to amplification processing,thereby generating a CDMA signal at the radio frequency band. Thegenerated CDMA signal is transmitted via the duplexer 4123 and theantenna 4101.

The OFDM transmission unit 4122B subjects the inputted transmission datato multi-carrier modulation in accordance with the OFDM scheme and toconversion into a radio frequency band as well as to amplificationprocessing, thereby generating an OFDM signal at the radio frequencyband. The generated OFDM signal is transmitted via the duplexer 4123 andthe antenna 4101.

The duplexer 4123 inputs the radio signal (either the CDMA signal or theOFDM signal) to the antenna 4101. Meanwhile, at the time of reception,the duplexer 4123 inputs the radio signal (either the CDMA signal or theOFDM signal) received by the antenna 4101 to the switch SW2.

The switch SW2 inputs a radio signal from the duplexer 4123 to any ofthe CDMA reception unit 4124A or the OFDM reception unit 4124B inaccordance with the control by the control unit 4160. The switch SW2inputs the radio signal from the duplexer 4123 to the CDMA receptionunit 4124A when the used transmission scheme is the CDMA scheme, orinputs a radio signal from the duplexer 4123 to the OFDM reception unit4124B when the used transmission scheme is the OFDM scheme.

The CDMA reception unit 4124A subjects the inputted radio signal (theCDMA signal) to conversion into the baseband and the amplificationprocessing, and performs inverse diffusion in accordance with the CDMAscheme. Moreover, the CDMA reception unit 4124A performs the RAKEreception which is the processing for combining the preceding wave andthe delayed waves included in the received CDMA signal. In the RAKEreception, the reception quality is improved by combining the precedingwave and the delayed waves while aligning the phases thereof. Thereception data thus obtained are inputted to the demodulation unit 4125.

The OFDM reception unit 4124B subjects the inputted radio signal (theOFDM signal) to conversion into the baseband and the amplificationprocessing, and performs multi-carrier demodulation in accordance withthe OFDM scheme. Moreover, the OFDM reception unit 4124B removes theguard intervals included in the received OFDM signal. The reception datathus obtained are inputted to the demodulation unit 4125.

The demodulation unit 4125 demodulates and decodes the inputtedreception data. The demodulation unit 4125 demodulates and decodes thereception data in accordance with a method corresponding to the certainmodulation scheme selected from the multiple modulation schemes.Moreover, the demodulation unit 4125 subjects the inputted receptiondata to the symbol determination.

The OFDM measurement unit 4141 measures reception parameters whichindicate the time difference between the preceding wave and the delayedwaves of the received OFDM signal. In the fifth embodiment, thereception parameters include an amplitude error and a phase errorbetween the OFDM symbol S included in the received OFDM signal and thereference point Sref of the OFDM symbol as shown in FIG. 34(a). Thevalues of the reception parameters (the amplitude error and the phaseerror) grow larger as the delay time difference Tdmax exceeds the guardinterval Tg by a larger degree. The reception parameters measured by theOFDM measurement unit 4141 are inputted to the control unit 4160.

The CDMA measurement unit 4142 measures the reception quality of theCDMA signal. In the fifth embodiment, the received signal strengthindicator (RSSI) is used as the reception quality of the CDMA signal.However, without limitation to the RSSI, it is also possible to use thereceived SNR (Signal to Noise ratio) and the like. The RSSI measured bythe CDMA measurement unit 4142 is inputted to the control unit 4160.

The control unit 4160 is formed by use of a CPU, for example, and isconfigured to control various functions incorporated in the radioterminal 4100. The storage unit 4180 is formed by use of a memory, forexample, and is configured to store a variety of information used forthe control by the control unit 4160.

The control unit 4160 controls the switches SW1, 2. The control unit4160 switches between the switches SW1, 2 when switching the usedcommunication scheme between the OFDM scheme and the CDMA scheme.Moreover, the control unit 4160 may temporarily switches between theswitches SW1, 2, when measuring the RSSI of the CDMA signal during theOFDM communication.

The control unit 4160 inputs the reception parameters measured by theOFDM measurement unit 4141 and the RSSI measured by the OFDM measurementunit 4141 to the modulation unit 4121. The OFDM transmission unit 4122Btransmits the reception parameters and the RSSI after the modulation tothe radio base station 4200. Such a report on the measurement results iscalled a “Measurement Report”.

The control unit 4160 operates the OFDM measurement unit 4141 and theCDMA measurement unit 4142 in response to the instruction from the radiobase station 4200, and executes the handover for switching from theradio base station 4200 to the radio base station 4300. Meanwhile, thecontrol unit 4160 performs the control so as to switch the connectiondestination from the radio base station 4300 (the CDMA base station) toa radio base station 4400 (illustrated in FIG. 38) serving as the OFDMbase station. Here, the radio base station 4200 means a first OFDM basestation that causes an intersymbol interference. Meanwhile, the radiobase station 4400 means a second OFDM station that does not cause anintersymbol interference.

(3) Configuration of Radio Base Station

FIG. 33 is a block diagram showing a configuration of the radio basestation 4200.

As shown in FIG. 33, the radio base station 4200 includes an antenna4201, a modulation unit 4221, a transmission unit 4222 (base-stationtransmission unit), a duplexer 4223, a reception unit 4224 (base-stationreception unit), a demodulation unit 4225, a control unit 4240, astorage unit 4260, and a wired communication unit 4280.

The modulation unit 4221 modulates and encodes the transmission datafrom the control unit 4240. The modulation unit 4221 modulates andencodes the transmission data based on the certain modulation schemeselected from the multiple modulation schemes in accordance with theadaptive modulation.

The transmission unit 4222 subjects the inputted transmission data tomulti-carrier modulation in accordance with the OFDM scheme and toconversion into a radio frequency band as well as to amplificationprocessing, thereby generating an OFDM signal at the radio frequencyband. The generated OFDM signal is transmitted via the duplexer 4223 andthe antenna 4201.

The duplexer 4223 inputs the OFDM signal to the antenna 4201. Meanwhile,at the time of reception, the duplexer 4223 inputs the OFDM signalreceived by the antenna 4201 to the reception unit 4224.

The reception unit 4224 subjects the inputted OFDM signal to conversioninto the baseband and the amplification processing, and performsmulti-carrier demodulation in accordance with the OFDM scheme. Moreover,the reception unit 4224 removes the guard intervals included in thereceived OFDM signal. The reception data thus obtained are inputted tothe demodulation unit 4225.

The demodulation unit 4225 demodulates and decodes the inputtedreception data. The demodulation unit 4225 demodulates and decodes thereception data in accordance with a method corresponding to the certainmodulation scheme selected from the multiple modulation schemes.Moreover, the demodulation unit 4225 subjects the inputted receptiondata to the symbol determination.

The control unit 4240 is formed by use of a CPU, for example, and isconfigured to control various functions incorporated in the radio basestation 4200. The storage unit 4260 is formed by use of a memory, forexample, and is configured to store a variety of information used forthe control by the control unit 4240. The wired communication unit 4280performs communication with the LTE network 4020 side.

The control unit 4240 transmits the measurement instruction for the RSSIof the CDMA signal to the radio terminal 4100 by using the transmissionunit 4222. In the fifth embodiment, the transmission unit 4222 transmitsthe measurement instruction either periodically or upon occurrence of apredetermined trigger.

The storage unit 4260 stores a neighbor list in advance, which includesinformation on radio base stations (hereinafter nearby base stations)located near the radio base station 4200. In the fifth embodiment, theneighbor list includes IDs, used channel information, and the like ofthe radio base stations (such as the radio base station 4300) supportingthe CDMA scheme. The control unit 4240 incorporates the neighbor list inthe measurement instruction and transmits the information from thetransmission unit 4222.

The control unit 4240 acquires the reception parameters and the RSSItransmitted from the radio terminal 4100 in response to the measurementinstruction via the reception unit 4224 and the demodulation unit 4225.The control unit 4240 determines whether or not to cause the radioterminal 4100 to perform the handover based on the reception parametersand the RSSI.

Specifically, the control unit 4240 calculates the EVM (Error VectorMagnitude) by use of the reception parameters and compares thecalculated EVM with the EVM threshold. The EVM threshold is determinedbased on the guard interval used in the OFDM scheme and is stored inadvance in the storage unit 4260.

The EVM threshold is preset to the value of the EVM when the delay timedifference Tdmax exceeds the guard interval length Tg. The value of theEVM at which the delay time difference Tdmax exceeds the guard intervallength Tg can be obtained on an experimental or an empirical basis.

In the fifth embodiment, the EVM threshold is provided for each of themodulation schemes to be used in the adaptive modulation. As shown inFIG. 34(a), the EVM is also referred to as the modulation accuracy,which represents an effective value of an error vector based on theamount of deviation (reception parameter) for the phase and theamplitude of the observed symbol point S from the symbol reference pointSref where the symbol point is supposed to be located and is expressedas square root percentage of average power of an ideal signal. FIG.34(c) shows a calculation formula for the EVM.

The storage unit 4260 stores a table which links the modulation schemeswith the EVM thresholds as shown in FIG. 34(b). A modulation schemewhich can achieve communication at a higher speed (a modulation schemeinvolving a larger bit number per symbol) has stricter restrictions onphase and amplitude errors. Accordingly, the EVM threshold is set to alower value as the modulation scheme can achieve communication at ahigher speed.

The control unit 4240 acquires the EVM threshold from the storage unit4260 based on the modulation scheme used for the downlink and comparesthe EVM threshold with the calculated EVM.

Moreover, the control unit 4240 compares the RSSI of the CDMA signalwith a predetermined value. When the RSSI is higher than thepredetermined value, it is possible to assume that the reception qualityis favorable. The predetermined value is preset to the value of the RSSIwith which the radio terminal 4100 can execute the communication.

Regarding the control unit 4240, when the calculated EVM exceeds the EVMthreshold and the RSSI is higher than the predetermined value, thecontrol unit 4240 of the radio base station 4200 decides to cause theradio terminal 4100 to execute the handover to the radio base station(such as the radio base station 4300) supporting the CDMA scheme. Whencausing the radio terminal 4100 to carry out the handover, the controlunit 4240 transmits an instruction for the handover to the radioterminal 4100 by use of the transmission unit 4222 after checking thatthe handover is feasible.

(4) Operations of Radio Communication System

Next, operations of the radio communication system 4010 will bedescribed by using (4. 1) Operation Pattern 1 and (4. 2) OperationPattern 2 as examples. In the fifth embodiment, the operations of theradio communication system 4010 are based on the 3GPPTS36.300 standardsand the like. The operation pattern 1 is an operation pattern configuredto cause the radio terminal 4100 to measure the RSSI of the CDMA signalperiodically. The operation pattern 2 is an operation pattern configuredto cause the radio terminal 4100 to measure the RSSI of the CDMA signalby using a certain event as a trigger.

(4. 1) Operation Pattern 1

FIG. 35 is a sequence chart showing an operation pattern 1 of the radiocommunication system 4010. This sequence is executed when the radioterminal 4100 is in an active scheme.

In step S4101, the transmission unit 4222 of the radio base station 4200transmits the measurement instruction to the radio terminal 4100. Theabove-described neighbor list is included in the measurementinstruction. The OFDM reception unit 4124B of the radio terminal 4100receives the measurement instruction.

In step S4102, the control unit 4160 of the radio terminal 4100 causesthe CDMA measurement unit 4142 to measure the RSSI of the CDMA signalfor each of the CDMA-supporting base stations corresponding to the IDsincluded in the neighbor list.

In step S4103, the control unit 4160 of the radio terminal 4100 causesthe OFDM measurement unit 4141 to measure the reception parameters ofthe OFDM signal received from the radio base station 4200.

In step S4104, the OFDM transmission unit 4122B of the radio terminal4100 transmits a measurement result report including the receptionparameters measured by the OFDM measurement unit 4141 and the RSSImeasured by the CDMA measurement unit 4142 to the radio base station4200. The reception unit 4224 of the radio base station 4200 receivesthe measurement result report (the reception parameters and the RSSI).

In step S4105, the control unit 4240 of the radio base station 4200calculates the EVM by use of the reception parameters which are receivedby the reception unit 4224 and demodulated by the demodulation unit4225.

In step S4106, the control unit 4240 of the radio base station 4200compares the calculated EVM with the EVM threshold corresponding to themodulation scheme. Meanwhile, the control unit 4240 of the radio basestation 4200 compares the RSSI, which is received by the reception unit4224 and demodulated by the demodulation unit 4225, with thepredetermined value.

When the calculated EVM exceeds the EVM threshold and the RSSI is higherthan the predetermined value, the control unit 4240 of the radio basestation 4200 decides to cause the radio terminal 4100 to execute thehandover to the CDMA-supporting radio base station (step S4107). On theother hand, when the calculated EVM falls below the EVM threshold orwhen the RSSI is equal to or below the predetermined value, the controlunit 4240 of the radio base station 4200 decides not to cause the radioterminal 4100 to execute the handover to the CDMA-supporting radio basestation.

Here, when there are multiple CDMA-supporting radio base stations havingthe RSSI higher than the predetermined value in the case where thecalculated EVM exceeds the EVM threshold, it is preferable that thecontrol unit 4240 of the radio base station 4200 determine theCDMA-supporting radio base station having the highest RSSI as a handoverdestination. In the following, a case of causing the radio terminal 4100to execute the handover to the radio base station 4300 will bedescribed.

In step S4108, the transmission unit 4222 of the radio base station 4200transmits a handover preparation instruction to the radio terminal 4100.The OFDM reception unit 4124B of the radio terminal 4100 receives thehandover preparation instruction.

In step S4109, the OFDM transmission unit 4122B of the radio terminal4100 transmits a request for connection to the radio base station 4300to the radio base station 4200. This request for connection is forwardedto the radio base station 4300 by tunneling under the supervision of theMME 4025 (step S4110). When the request for connection is successful,the MME 4025 notifies the radio base station 4200 of the fact (stepS4111).

In step S4112, the radio base station 4200 transmits an instruction forthe handover to the radio base station 4300 to the radio terminal 4100in response to the notification from the MME 4025. Upon receipt of theinstruction for the handover to the radio base station 4300, the radioterminal 4100 executes the handover to the radio base station 4300.

(4. 2) Operation Pattern 2

FIG. 36 is a sequence chart showing an operation pattern 2 of the radiocommunication system 4010. This sequence is executed when the radioterminal 4100 is in an active scheme.

In step S4201, the control unit 4160 of the radio terminal 4100 causesthe OFDM measurement unit 4141 to measure the reception parameters ofthe OFDM signal received from the radio base station 4200.

In step S4202, the OFDM transmission unit 4122B of the radio terminal4100 transmits the reception parameters measured by the OFDM measurementunit 4141 to the radio base station 4200. The reception unit 4224 of theradio base station 4200 receives the reception parameters.

In step S4203, the control unit 4240 of the radio base station 4200calculates the EVM by use of the reception parameters which are receivedby the reception unit 4224 and demodulated by the demodulation unit4225.

In step S4204, the control unit 4240 of the radio base station 4200compares the calculated EVM with the EVM threshold corresponding to themodulation scheme.

When the calculated EVM exceeds the EVM threshold, the transmission unit4222 of the radio base station 4200 transmits a RSSI measurementinstruction to the radio terminal 4100 in step S4205. Theabove-described neighbor list is included in the measurementinstruction. The OFDM reception unit 4124B of the radio terminal 4100receives the measurement instruction.

In step S4206, the control unit 4160 of the radio terminal 4100 causesthe CDMA measurement unit 4142 to measure the RSSI of the CDMA signalfor each of the CDMA-supporting base stations corresponding to the IDsincluded in the neighbor list.

In step S4207, the OFDM transmission unit 4122B of the radio terminal4100 transmits the measurement result report including the RSSI measuredby the CDMA measurement unit 4142 to the radio base station 4200. Thereception unit 4224 of the radio base station 4200 receives themeasurement result report (the RSSI).

In step S4208, the control unit 4240 of the radio base station 4200compares the RSSI, which is received by the reception unit 4224 anddemodulated by the demodulation unit 4225, with the predetermined value.

When the RSSI is higher than the predetermined value, the control unit4240 of the radio base station 4200 decides to cause the radio terminal4100 to execute the handover to the CDMA-supporting radio base station(step S4209). On the other hand, when the RSSI is equal to or below thepredetermined value, the control unit 4240 of the radio base station4200 decides not to cause the radio terminal 4100 to execute thehandover to the CDMA-supporting radio base station. Here, when there aremultiple CDMA-supporting radio base stations having the RSSI higher thanthe predetermined value, it is preferable that the control unit 4240 ofthe radio base station 4200 determine the CDMA-supporting radio basestation having the highest RSSI as the handover destination.

Each processing in steps S4210 to S4214 is executed as similar to theoperation pattern 1.

(4. 3) Operation Pattern 3

In this pattern, when the control unit 4160 of the radio terminal 4100temporarily switches from connection to the radio base station 4200 (theOFDM radio base station) to connection to the radio base station 4300(the CDMA radio base station) in light of a data transmission capacityand then determines that the radio terminal 4100 moves to an area (adesired area) predicted to achieve a higher data transmission capacityby communication through the base station in the OFDM scheme thancommunication through the CDMA radio base station, the control unit 4160controls so as to switch to connection to the radio base station 4400(the OFDM radio base station in FIG. 38).

FIG. 38 is a sequence chart showing an operation pattern 3 of the radiocommunication system 4010. This sequence is executed when the radioterminal 4100 is in an active scheme.

In step S4301, the radio base station 4300 transmits the measurementinstruction to the radio terminal 4100. The above-described neighborlist is included in the measurement instruction. The CDMA reception unit4124B of the radio terminal 4100 receives the measurement instruction.

In step S4302, the control unit 4160 of the radio terminal 4100 causesthe CDMA measurement unit 4142 to measure the RSSI of the CDMA signalfor each of the CDMA-supporting base stations corresponding to the IDsincluded in the neighbor list.

In step S4303, the control unit 4160 of the radio terminal 4100 causesthe OFDM measurement unit 4141 to measure the RSSI of the OFDM signalreceived from the radio base station 4200.

In step S4304, the control unit 4160 determines that the radio terminal4100 has moved, when a reception intensity (the RSSI of the radio basestation 4300 among the RSSI of the CDMA signal measured by the CDMAmeasurement unit 4142) at the present moment becomes smaller than areception intensity at the time of switching to connection to the radiobase station 4300 (the CDMA radio base station). Based on the fact thatthe reception intensity at the present moment becomes smaller, the radioterminal 4100 presumably moves away from the radio base station 4300,and is expected to have moved to an area where the radio terminal 4100can achieve the higher data transmission capacity by performingcommunication through the base station in the OFDM scheme than byperforming communication through the CDMA radio base station.

In step S4305, the control unit 4160 transmits the measurement resultreport including the RSSI of the OFDM signal measured by the OFDMmeasurement unit 4141 and the RSSI of the CDMA signal measured by theCDMA measurement unit 4142 to the radio base station 4300. Here, whenthe control unit 4160 determines that the radio terminal 4100 has moved,the fact of the movement of the radio terminal 4100 is set to themeasurement result report by the control unit 4160.

In step S4306, the radio base station 4300 receives this measurementresult report and compares the RSSI of the OFDM signal with the RSSI ofthe CDMA signal.

In step S4307, the radio base station 4300 decides to cause the radioterminal 4100 to execute the handover to the OFDM-supporting radio basestation 4400 when the highest RSSI of the OFDM signal is greater thanthe highest RSSI of the CDMA signal and when the fact of the movement ofthe radio terminal 4100 is set to the measurement result report.

In step S4308, the radio base station 4300 transmits the handoverpreparation instruction to the radio terminal 4100. The CDMA receptionunit 4124A of the radio terminal 4100 receives the handover preparationinstruction.

In step S4309, the CDMA reception unit 4124A of the radio terminal 4100transmits the request for connection to the radio base station 4400 tothe radio base station 4300. This request for connection is forwarded tothe radio base station 4400 by tunneling via the SGSN/MME 4025 (stepS4310). When the request for connection is successful, the SGSN/MME 4025notifies the radio base station 4300 of the fact (step S4311).

In step S4312, the radio base station 4300 transmits the instruction forthe handover to the radio base station 4400 to the radio terminal 4100in response to the notification from the MME 4025. Upon receipt of theinstruction for the handover to the radio base station 4400, the radioterminal 4100 executes the handover to the radio base station 4400.

In the operation pattern 3, the control unit 4160 of the radio terminal4100 is configured to determine that the radio terminal 4100 has movedwhen the reception intensity at the present moment becomes smaller thanthe reception intensity at the time of switching to connection to theradio base station 4300 (the CDMA radio base station). Instead, thecontrol unit 4160 may be configured to acquire the current communicationarea from the radio base station 4300 (the CDMA radio base station) andto determine that the radio terminal 4100 has moved if identificationinformation of the communication area at the time of switching to theradio base station 4300 does not coincide with identificationinformation of the acquired current communication area. Here, theidentification information of the communication area may be designed toidentify a location registration area. If a GPS is loaded on the radioterminal 4100, the identification information of the communication areamay be locational information on the own radio terminal acquired by theGPS.

Meanwhile, although the operation pattern 3 is described as theembodiment configured to execute the handover to the radio base station4400, there is also a possibility to execute the handover again to theradio base station 4200 depending on the magnitude of the signalintensity of the OFDM signal and the like.

In the operation pattern 3, after switching from the OFDM scheme to theCDMA scheme due to the intersymbol interference, the radio terminal 4100determines that the radio terminal 4100 has moved. This determinationprevents the radio terminal 4100 from communicating in the OFDM schemein the same location as the location at the time point of switching tothe CDMA scheme. Accordingly, even when the CDMA scheme is switched tothe OFDM scheme again, deterioration in the communication performancedue to the intersymbol interference is avoided.

(5) Effect of Fifth Embodiment

According to the fifth embodiment, the control unit 4240 of the radiobase station 4200 decides to cause the radio terminal 4100 to executethe handover to the radio base station 4300 when the EVM exceeds the EVMthreshold and the RSSI is higher than the predetermined value. Here, theEVM reflects the delay time difference Tdmax between the preceding waveand the delayed waves of the OFDM signal and the EVM threshold is set tothe value of the EVM when the delay time difference Tdmax exceeds theguard interval length Tg. The state of the EVM exceeding the EVMthreshold means that the delay time difference Tdmax exceeds the guardinterval length Tg.

Accordingly, under the situation where the delay time difference Tdmaxis estimated to exceed the guard interval length Tg, it is possible toavoid an intersymbol interference and to avoid deterioration in thecommunication performance by causing the radio terminal 4100 to executethe handover to the radio base station 4300 supporting the CDMA schemeafter checking that a reception level of the CDMA signal is favorable.Moreover, it is possible to utilize the excellent communicationperformance of the OFDM scheme until the delay time difference Tdmax isestimated to exceed the guard interval length Tg.

Therefore, the radio terminal 4100 supporting both of the CDMA schemeand the OFDM scheme can avoid deterioration in the communicationperformance due to the intersymbol interference while utilizing the OFDMscheme in the active scheme.

Furthermore, the CDMA reception unit 4124A of the radio terminal 4100can perform the RAKE reception in which the preceding wave and thedelayed waves in the CDMA signal are combined. Accordingly, it ispossible to achieve high resistance to the multi-path environment and toobtain a path diversity effect by the RAKE reception. Therefore, it ispossible to utilize the characteristic of the CDMA scheme and toeffectively suppress deterioration in the communication performance byswitching the used communication scheme from the OFDM scheme to the CDMAscheme.

Meanwhile, the EVM (and reception parameter) has the advantageousfeatures that it can be measured with a smaller amount of operation andthat a shorter time is required for the measurement as compared to otherreception quality indices including the SNR (Signal to Noise ratio), theBER (Bit Error Rate), the channel estimation value, and the like. Forthis reason, by using the EVM, it is possible to estimate whether or notthe delay time difference Tdmax exceeds the guard interval length Tgeasily and instantaneously. Therefore, it is possible to reduce aprocessing load and power consumption of the radio terminal 4100 ascompared to the case of using other reception quality indices, and toshorten a period of deterioration in the communication performance dueto the intersymbol interference.

In the fifth embodiment, the control unit 4240 sets the EVM thresholdcorresponding to the modulation scheme used for the downlink radiocommunication. Accordingly, it is possible to set the EVM thresholdvalue appropriately even when using the adaptive modulation.

In the case of the radio base station and the radio terminalconstituting the radio communication system of the fifth embodiment, theradio terminal determines that the radio terminal has moved afterswitching from the OFDM scheme to the CDMA scheme due to the intersymbolinterference, and does not communicate in the OFDM scheme in the samelocation as the location at the time point of switching to the CDMAscheme. Accordingly, deterioration in the communication performance dueto the intersymbol interference is avoided even when the CDMA scheme isswitched to the OFDM scheme again.

(6) Modified Examples of Fifth Embodiment

It is also possible to measure the reception parameters of the OFDMsignal only when the state of the voltage waveform of the received OFDMsignal satisfies a certain condition. FIG. 37 shows waveform measurementprocessing by the OFDM measurement unit 4141. In the example of FIG. 37,the delay time difference is equal to T as shown in FIG. 37(a), and thepreceding wave shown in FIG. 37(b) is combined with the delayed waveshown in FIG. 37(c), and the OFDM signal shown in FIG. 37(d) isreceived. The OFDM measurement unit 4141 specifies the guard intervalperiod of the preceding wave based on a result of symbolsynchronization, for example, and measures the state of the voltagewaveform (a voltage value) of the OFDM signal at measurement timingcorresponding to the guard interval period. The measurement is performedat each measurement timing corresponding to each of the guard intervalperiods. The OFDM measurement unit 4141 determines whether or not thestate of the voltage waveform measured at the current measurement timingT(n) is equal to the state of the voltage waveform measured at themeasurement timing precedent to the current measurement timing(hereinafter referred to as the “precedent measurement timing”) T(n−1).The OFDM measurement unit 4141 omits the measurement of the receptionparameters when the state of the voltage waveform measured at thecurrent measurement timing T(n) is equal to the state of the voltagewaveform measured at the precedent measurement timing T(n−1), orexecutes the measurement of the reception parameters when the state ofthe voltage waveform measured at the current measurement timing T(n) isdifferent from the state of the voltage waveform measured at theprecedent measurement timing T(n−1). In the case of performing thedetermination only by use of the EVM, there is a risk of erroneousdetermination if the EVM varies due to a factor (such as a circuitfactor) other than a change in the multipath state. Accordingly, it ispossible to improve determination accuracy by measuring the EVM onlywhen the state of the voltage waveform measured at the currentmeasurement timing T(n) is different from the state of the voltagewaveform measured at the precedent measurement timing T(n−1).

The fifth embodiment has described the example of individually providingthe CDMA communication unit (the CDMA transmission unit 4122A and theCDMA reception unit 4124A) and the OFDM communication unit (the OFDMtransmission unit 4122B and the OFDM reception unit 4124B). However, itis also possible to use an aspect of forming the CDMA communication unitand the OFDM communication unit collectively as one communication unit.For example, in a radio terminal called a cognitive terminal, it ispossible to switch the communication scheme by way of software bydownloading software (SDR BB, Tunable RF) corresponding to the usedcommunication scheme.

In the fifth embodiment, the reception parameters to be measured by theradio terminal 4100 include the amplitude error and the phase error andthe EVM is calculated by the radio base station 4200. However, the radioterminal 4100 may calculate the EVM by use of the amplitude error andthe phase error and transmit the EVM as the reception parameter to theradio base station 4200. In this case, the “value corresponding to thereception parameter” is the value of the EVM. Alternatively, withoutlimitation to the case of using the EVM, it is also possible to useanother reception quality index (such as the SNR, the BER or the channelestimation value).

In the fifth embodiment, the cellular phone terminal is shown as theexample of the radio terminal 4100. However, without limitation to thecellular phone terminal, it is possible to use a terminal equipped withcommunication devices according to the CDMA scheme and the OFDM scheme,for example.

Although the fifth embodiment has described the example in which theguard interval length is a fixed length, the guard interval length mayalso be a variable length. For example, in the case of selectively usingtwo types of guard intervals, namely, a short guard interval and a longguard interval longer than the short guard interval, it is preferable tomeasure the reception parameters (or the EVM) at the time of the longguard interval from the viewpoint of ensuring the measurement time.

In the fifth embodiment, the LTE has been described as the example ofthe radio communication system employing the OFDM scheme. However,without limitation to the LTE, it is also possible to use WiMAXstandardized by IEEE 802.16, a next-generation PHS (XGP), and the like.

Note that the entire content of Japanese Patent Application No.2009-77744 (filed on Mar. 26, 2009), Japanese Patent Application No.2009-127064 (filed on May 26, 2009), Japanese Patent Application No.2009-127068 (filed on May 26, 2009), and Japanese Patent Application No.2009-269471 (filed on Nov. 27, 2009) is incorporated herein byreference.

INDUSTRIAL APPLICABILITY

As described above, a radio terminal, a radio communication system, anda radio base station according to the present invention can preventdeterioration in a communication performance due to an intersymbolinterference, and are therefore useful for radio communication such asmobile telecommunication.

The invention claimed is:
 1. A radio terminal comprising: a transceiverthat performs radio communication by selecting any of a Code DivisionMultiple Access (CDMA) scheme and an Orthogonal Frequency DivisionMultiplexing (OFDM) scheme as a used communication scheme to be used ina cell formed by a radio base station; and a processor that controlsmeasuring reception quality of a reception signal of the OFDM scheme andmeasuring reception quality of a reception signal of the CDMA scheme;wherein, while the OFDM scheme is selected as the used communicationscheme, the processor controls measuring the reception quality of thereception signal of the CDMA scheme only when both the user terminal isin an idle state and the reception quality measured in the OFDM schemeis less than a first threshold used in the OFDM scheme, and wherein,when both the user terminal is in the idle state and a reception qualitymeasured in the CDMA scheme is greater than a second threshold, theprocessor controls reselecting a cell formed by a radio base stationsupporting the CDMA scheme, and controls the transceiver to cause thetransceiver to switch the used communication scheme from the OFDM schemeto the CDMA scheme.
 2. The radio terminal according to claim 1, whereinwhen adaptive modulation is used for the radio communication, theprocessor sets the first threshold based on a modulation scheme used forthe radio communication.
 3. A radio terminal comprising: a transceiverthat performs radio communication by selecting any of a Code DivisionMultiple Access (CDMA) scheme and an Orthogonal Frequency DivisionMultiplexing (OFDM) scheme as a used communication scheme to be used ina cell formed by a radio base station; one or more processors; memory;and one or more modules stored in the memory and configured forexecution by the one or more processors, the one or more modulesincluding instructions to measure reception quality of a receptionsignal of the OFDM scheme, while the OFDM scheme is selected as the usedcommunication scheme, measure reception quality of a reception signal ofthe CDMA scheme only when both the OFDM scheme is in an idle state andthe reception quality measured in the OFDM scheme is less than a firstthreshold used in the OFDM scheme, and, when both the OFDM scheme is inthe idle state and a reception quality measured in the CDMA scheme isgreater than a second threshold, control reselecting a cell formed by aradio base station supporting the CDMA scheme, and control thetransceiver to cause the transceiver to switch the used communicationscheme from the OFDM scheme to the CDMA scheme.
 4. A communicationcontrol method used in a radio terminal, the method comprising:performing, by the radio terminal, radio communication by selecting anyof a Code Division Multiple Access (CDMA) scheme and an OrthogonalFrequency Division Multiplexing (OFDM) scheme as a used communicationscheme to be used in a cell formed by a radio base station; measuring,by the radio terminal, reception quality of a reception signal of theOFDM scheme; while the OFDM scheme is selected as the used communicationscheme, measuring, by the radio terminal, reception quality of areception signal of the CDMA scheme only when both the OFDM scheme is inan idle state and the reception quality obtained by the radio terminalin the OFDM scheme is less than a first threshold used in the OFDMscheme; and, when both the OFDM scheme is in the idle state and areception quality measured in the CDMA scheme is greater than a secondthreshold, reselecting a cell formed by a radio base station supportingthe CDMA scheme, and switching the used communication scheme from theOFDM scheme to the CDMA scheme.